You're not going to like my response. :P
It KINDA sounds like you're jumping into a new relationship a bit too quickly, and looking to this new girl to help you get over your feelings for your ex, which is unfair to her. Two months of single life is very short in comparison to a three year relationship, especially with leftover feelings. In my opinion, your ex doesn't deserve another chance (her "I was just depressed" explanation kinda sounds like a load of crap IMO). What you've deemed as sincerity as far as wanting you back doesn't necessarily mean she's going to be the girlfriend you need her to be. She's very likely realized how good she had it with you, and wants that back. That doesn't mean she's going to suddenly stop fooling around, start growing up, and treat you properly. She needs to prove that she can function as a human being by herself, without using you as a crutch (Do you really want the drama of a girl who drinks, parties, and quits school as a way of avoiding problems?). This new girl sounds better for you, but as I said, it's not fair to start a relationship with her while you still love your ex so much.
Plain and simple... you're young, and it's very likely that neither of these girls is the one you'll end up with for the rest of your life, or even 5 years from now, so there's no real reason to rush a decision. Like YH, I feel that if either of these girls was the right answer, you wouldn't be so conflicted right now... It might be best to at least tell the new girl you need to step back for a bit, because you NEED to deal with your conflicting feelings before you can truly commit to anyone. Honestly, you need to make sure you're happy with yourself before being happy with someone else. I know that feeling you get after the end of a long-term relationship where you just want to jump into a new relationship because you miss the benefits and companionship, but that's really not the best way to deal with the situation.
Anyway, good luck with whatever you decide.