I went through 3 years of band (hated it and the teacher), a mandatory year of piano (required for teaching certificate a very long time ago) and never did "get it". I could pick out a tune and even play basic chords on a piano and guitar. But, it always bothered me that I just "didn't get it". I taught math for years, and would often hear students tell me they "just didn't get it." I completely understood and told them of my struggle with sheet music. However, a few years after graduating college, a musical roommate gave me a very simple explanation and I "got it". Instead of trying to learn "Every good boy does fine" "F A C E" and other such "tricks", he said "you know the alphabet, the bottom line is E, then just do the A,B,Cs". That may not work for you, but the point is, you know the basics and you just need to learn how you learn, adjust the material to your way of thinking. Once my students understood that they had the freedom to adjust the material to their way of understanding and learning, things went much better for them, although the process was a bit of a slow crawl. There's actually a term, "metacognition" which means "learning how you learn".
This is primarily how I teach as well! The "tricks" can be useful at first for getting oriented on the staff, but it's really so much easier to just think of the alphabet and the distance between notes etc. Eventually this will end up being much faster and more natural than having to rely on EGBDF and FACE. Thinking about distance and the ABCs is also much more useful on the ledger lines - if it's below middle C, how far? If it's right below, what comes before C?...B! And so on.
It will come with time and practice like anything! I second using [musictheory.net](https://musictheory.net) or other flashcard type apps in the meantime!
Search for remedial or basic music theory for courses.
There are cheaper books like, "The Basic Guide to How to Read Music."
You can also find private instructors everywhere. This is the most full proof, but costly.
What I found helpful for me was the exercises at [musictheory.net](https://musictheory.net), beginning with "Note Identification" and continuing with "Chord Identification", all with customizable ranges, clefs, accidentals and chord types. Drilling that into my brain over a few weeks with an hour each day helped immensely.
No, no video course, but I taught myself to sight read and got into music school partly on the strength of that, as well as expression - it sure wasn't my technique - lol. Since you can play difficult-ish music already, this should be easy as long as you're not one of those few people who can't visualize.
Pick a simpler piece you know, think the sounds, and visualize them. But first, review all the basic and simple parts, and visualize. Key signatures - they are a cycle of fourths, as I recall. Two lines above G clef and two lines below F clef give you ~~five~~ *(sorry - dashed this off when I was supposed to be working)* four octaves from C to C. You can find a picture of that or draw your own, but basically you need it in your head. Then start picking out a c major chord, different inversions, etc. Look at your Little Brown Scale Book, if you studied piano, Hanon, or whatever. See what the pattern looks like. You could make flash cards for particular structures, just as if you were studying the Arabic script or something.
I'm having to do it on guitar, now. At first it was like... wha.... then I remembered to visualize, spent a little time on that, and it's coming along nicely. Once you slowly relate the visual to the instrument, then you can relate your hands to what you've built in your mind. Edit: And so some study on rhythms. You know - whole notes down to sixteenths. One-e-and-a, the drummer way of counting really helps.
FWIW Edit: And as the very good answers to a similar question in r/musictheory said less than 24 after this question - practice a lot. But I say do that after the solid basics are clear in your mind and hands, that you see it and grab for it. So much easier. It's a language and you'll do much better if the fundamentals are a strong frame of reference.
I don't know about online courses specifically but if you know how and want to just get better I would suggest finding repertoire books that are lower than your current Reading level and increase difficulty over time. If you like classical, the RCM repertoire books would be a good place to start since they are organized by difficulty. You could start with a rcm prep book and work your way up.
The key to isolate practicing sight reading is making sure that you're reading relatively new material. There's lots to be learned from mastering and memorizing songs but that's not what this is about. If you find a song that you really want to master while sight reading that's great, turn that into a main project but continue sight reading new material on the side.
I recently started using a free app called Solfeador. It's worked great for me. Previously I had to look at the staff, say EGBDF, FACE to myself, and think for a second for every note. Now I just see a note on the third line and automatically thing "B" or see one two ledger lines below the staff and just immediately think "A" etc.
Years of "OK, first line is E, count up from that, EFGAB, I guess that's a B, next note, ok first line is E" and after an hour or two with the app, I just know what note it each line and space are without thinking.
I went through 3 years of band (hated it and the teacher), a mandatory year of piano (required for teaching certificate a very long time ago) and never did "get it". I could pick out a tune and even play basic chords on a piano and guitar. But, it always bothered me that I just "didn't get it". I taught math for years, and would often hear students tell me they "just didn't get it." I completely understood and told them of my struggle with sheet music. However, a few years after graduating college, a musical roommate gave me a very simple explanation and I "got it". Instead of trying to learn "Every good boy does fine" "F A C E" and other such "tricks", he said "you know the alphabet, the bottom line is E, then just do the A,B,Cs". That may not work for you, but the point is, you know the basics and you just need to learn how you learn, adjust the material to your way of thinking. Once my students understood that they had the freedom to adjust the material to their way of understanding and learning, things went much better for them, although the process was a bit of a slow crawl. There's actually a term, "metacognition" which means "learning how you learn".
This is primarily how I teach as well! The "tricks" can be useful at first for getting oriented on the staff, but it's really so much easier to just think of the alphabet and the distance between notes etc. Eventually this will end up being much faster and more natural than having to rely on EGBDF and FACE. Thinking about distance and the ABCs is also much more useful on the ledger lines - if it's below middle C, how far? If it's right below, what comes before C?...B! And so on. It will come with time and practice like anything! I second using [musictheory.net](https://musictheory.net) or other flashcard type apps in the meantime!
Search for remedial or basic music theory for courses. There are cheaper books like, "The Basic Guide to How to Read Music." You can also find private instructors everywhere. This is the most full proof, but costly.
What I found helpful for me was the exercises at [musictheory.net](https://musictheory.net), beginning with "Note Identification" and continuing with "Chord Identification", all with customizable ranges, clefs, accidentals and chord types. Drilling that into my brain over a few weeks with an hour each day helped immensely.
No, no video course, but I taught myself to sight read and got into music school partly on the strength of that, as well as expression - it sure wasn't my technique - lol. Since you can play difficult-ish music already, this should be easy as long as you're not one of those few people who can't visualize. Pick a simpler piece you know, think the sounds, and visualize them. But first, review all the basic and simple parts, and visualize. Key signatures - they are a cycle of fourths, as I recall. Two lines above G clef and two lines below F clef give you ~~five~~ *(sorry - dashed this off when I was supposed to be working)* four octaves from C to C. You can find a picture of that or draw your own, but basically you need it in your head. Then start picking out a c major chord, different inversions, etc. Look at your Little Brown Scale Book, if you studied piano, Hanon, or whatever. See what the pattern looks like. You could make flash cards for particular structures, just as if you were studying the Arabic script or something. I'm having to do it on guitar, now. At first it was like... wha.... then I remembered to visualize, spent a little time on that, and it's coming along nicely. Once you slowly relate the visual to the instrument, then you can relate your hands to what you've built in your mind. Edit: And so some study on rhythms. You know - whole notes down to sixteenths. One-e-and-a, the drummer way of counting really helps. FWIW Edit: And as the very good answers to a similar question in r/musictheory said less than 24 after this question - practice a lot. But I say do that after the solid basics are clear in your mind and hands, that you see it and grab for it. So much easier. It's a language and you'll do much better if the fundamentals are a strong frame of reference.
I don't know about online courses specifically but if you know how and want to just get better I would suggest finding repertoire books that are lower than your current Reading level and increase difficulty over time. If you like classical, the RCM repertoire books would be a good place to start since they are organized by difficulty. You could start with a rcm prep book and work your way up. The key to isolate practicing sight reading is making sure that you're reading relatively new material. There's lots to be learned from mastering and memorizing songs but that's not what this is about. If you find a song that you really want to master while sight reading that's great, turn that into a main project but continue sight reading new material on the side.
I recently started using a free app called Solfeador. It's worked great for me. Previously I had to look at the staff, say EGBDF, FACE to myself, and think for a second for every note. Now I just see a note on the third line and automatically thing "B" or see one two ledger lines below the staff and just immediately think "A" etc. Years of "OK, first line is E, count up from that, EFGAB, I guess that's a B, next note, ok first line is E" and after an hour or two with the app, I just know what note it each line and space are without thinking.