I've played guitar for...god I think going on 12 years now? And I still can't even play 'rhythm' or 'lead'. I have relatively high technical skill, I can play Canon Rock for example: http://www.youtube.com/watch?v=2xjJXT0C0X4 And if I wanted to compose my own arrangement, I could sit down and do that without much trouble. But if you asked me to improvise something, ANYTHING, even just a basic chord progression, I cannot.

It's because I don't know the first thing about music. So my advice would be to study composition, because the technical skill required to play lead simply comes with practicing lead lines. However if all you do is copy licks from tabs or whatever (like I do) you'll never really learn. When people ask me if I play an instrument I always qualify it and say I don't so much play it as I do screw around on it.

P.S. if you can sing and play you've already got me beat. That's one of those things some people can just never get.