[Bring_Back_Shantz]

Okay, not be be a dick, but it seems to me that you broke up with her and now regret that decision.

Every time I hear a story even remotely like this, there is always the part about, "she deserves someone better", which usually means "She deserves me".

She's obviously got some problems, and she is coming back to you for help because she knows she can.
She's moved on, you on the other hand are experiencing some "Seller's Remourse".
Maybe it's time for some tough love where you tell her to call a cab, and maybe you should help her out when she calls you at 4:00 AM, but don't do it because you're still hung up on her, you broke up with her so that ship has probably passed.

If you do help her thinking you'll be able to get her back, you're just letting her use you, because chances are you'll submit to just about any unreasonable request and she'll still be able to go off with Johnny Not-so-nice.

But if you help her because she is a friend who need help, you can be objective about it and when she calls for a ride at 4 AM and then goes back to the guy, you can tell her she's being ######ed, and next time she can call a cab (ie, tell her to help herself).