Find a teacher, I'm guilty of not doing this myself but I would be the first person to recommend you find one. If they're good they'll be able to guide you in the right direction, I'm sure of it.

If not, check out this guy's videos he's a fantastic teacher.

Pretty much what fallingleaves said, right down to me being guilty of not finding a teacher. If you can't find a teacher, I usually find that getting some method/theory books gets me excited about playing again. Otherwise, I'm not sure I have much advice for you. I don't consider myself that great at it, and I often feel like I'm at a wall. I had a reputation for being really good among my friends because I learned a bunch of flashy songs from the Candyrat Records crowd, but not all of them are that hard. It's more a matter of learning an uncommon technique. Actually, look up 'Song for Stephen' by Antoine Dufour - it's not too difficult, has an awesome 'party trick' in the middle of it that looks hard but is actually pretty easy to learn, and usually blows listeners away (you can buy tablature from Candyrat's website). It won't necessarily make you better, but it will make you different, and sometimes getting out of a rut is just as good.

Otherwise, yeah, get lessons, get books, watch youtube videos - try to find inspiring stuff, and don't quit. Again, I don't think I'm that great, so take my advice with a grain of salt - I really don't think it matters what you do. Don't worry about finding the 'best' exercise. Just find an exercise that does what you want and stick with it. The important part is to keep doing things, regardless of whether you think you're improving. It takes time and gradual change is hard to see. I've never had the patience to do it, but try recording yourself every few days and comparing the recordings to see how you improve.

If you haven't done it yet, try learning to play scales by ear. It's a great exercise, and it changed me from being completely unable to improvise into someone who can improvise poorly (hey, something's better than nothing, right?). Pick a major scale pattern. Play it over and over until you have the sound of it stuck in your head, then play it one fifth higher (seven frets up one string, or two frets up and one string over, unless you're on the g & b strings, in which case it's three frets up) without looking up a new pattern. Try to figure it out by ear. When you've got that done, try going up another fifth (or down a fourth - it's equivalent. Down 5 frets on one string, or keep on the same fret and go one string lower). Keep this up until you get back to where you started. If you get bored, try watching TV while you do it. It'll probably slow you down, but so what? As long as you keep practicing, right?

Also, learn some music theory if you haven't already. This is the most approachable guide I've ever seen, and it goes into a fair bit of depth. And try writing your own music. Record stuff, overdub it, just have some fun. You don't even have to share it with anyone if it's not up to your standards.