Hi, I'm 17, I stopped playing guitar like 4 years ago(wasn't very good) and now I want to play again , I've been playing for 1 month and I can see that i'm getting better but i'm tired of playing easy riff such as metallica and not being able to play solo in the song... It would be nice if you guys could tell me what songs would have heavier riff but not too heavy solo...
Maybe look at Five Finger Death Punch. They aren't particularly technical, but the riffs are kind of challenging.
You might want to take and step out of your realm of comfort occassionally. Grab something like TuxGuitar and pick out a portion of a solo that you might not be able to handle. Slow it down till you've got a chance to get your accuracy right, practice it.

Check this site out might help you, http://www.myguitarsolo.com/ there are some nice things to take away from it. The licks are neat, don't forget to slow things down though and gradually bring them up to speed. Better to be slow and accurate than quick not playing it perfectly. Plus it will expand your comfort with what you can do.

Kudos chief