I've been playing for about 3 years, and when I jam to a song by following the chords and coming up with my own lines, I can back up a track pretty solidly as far as rhythm/groove. But when I actually try to learn a specific line or solo, all my shortcomings come flying onto my face. For example, I just tried to learn "Good Times Bad Times" by Led Zep, and can't even get past 15 seconds without completely tripping up. It's killing me 'cause I can never bring myself to join a band or anything because I'd have to learn a bunch of songs and find I can't handle them technically. What are some exercises I can do to improve my speed/dexterity and switching between trickier intervals, that'll allow me to take on at least most modern rock songs with ease?