I had this problem when I was learning the solo in Dead Skin Mask by Slayer. What I did was I dropped the tempo down drastically, I'm talkin like 80 BPM (painfully slow) then took it lick by lick. Before learning a new lick I would get the fingering of it memorized to the point that it was muscle memory. Once you get the muscle memory down, increase the tempo in small increments, like by 5 or 10 BPM until it gets to the point that you can barely do it, then go back down to a slower level again. Keep pushing yourself to that "breaking point" but dont go past it. If you keep doing that it will help ALOT with getting that solo up to speed. I've even pushed it past the song speed a few times, just to make the actual speed seem slower and easier to play, so you should consider doing that too. Also something that really helps is listening to the solo while you try to play along, because that can help with the way that the notes are played.