Interestingly, I've never been able to read music either, I can SORT-OF learn tab, although I am very poor at remembering it, and while actively being instructed it takes too long, takes me hours to figure out, note by note, (guitar/bass) just one or two bars at most. I gave up eventually with my guitar classes, used to take some 1-to-1 lessons with an instructor that lived relatively locally, but fucked it off, because I was more or less just wasting his time, not out of any desire to piss him about, but because whilst I was there, and he could actually show me the fret positions and fingering patterns, that I could do, but when the lessons were over, I was completely unable to work on anything any further than what I had just been shown, leaving me with no way to make any progress.
Its to do with learning and perception of the mathematical concepts behind the learning of music and its notation, rather than any effect on playing ability, as when someone has taken the time to show me, bit by bit, the notes to play, I've been told I'm actually pretty good. Or rather, used to be, I ended up pawning my guitar, bass and amp, knowing I'd never be able to progress any further than I have already.