Banks listen up cleary for a second. The first thing you need to tell yourself is that you are NOT over her. If you were, you would NOT have acted that way the first moment you saw her. There is no point in lying to yourself and attempting to convince yourself how you feel. In actuality you know you do have feelings for her -- let them be lust or love, or, infatuation. Certain types of infatuation are just 'one person loving the other' without the other reciprocating that love.
Anyways, I just wanted to clear that issue up first, because, once you realize and truly admit how you feel about this girl, the answer should come to you without any static. It will be picture-perfect. If you truly love her (or feel you do) and after all that time of not seeing her, you just fall right back into the same 'hole' you were in before, then there MUST be something special about that girl. I say, go for it man. Once she breaks your heart, then you can say, "I should never have." But until she does (maybe she never would), it could be the best time of your life. If you never try to see what happens between you two, you could (most indubitably I would think) regret it for the rest of your entire life. I say, "Why leave such a burden on yourself? Just do it."
"Live as if you were to die tomorrow. Learn as if you were to live forever."
- Gandhi.
About the other guy. Don't sweat it. Maybe she realized you are a way better guy then that jerk (hehe) could ever have been. Maybe you just got hurt by seeing her close to someone else, and blew it out of proportion? That could be why you 'beat' your feelings for her down, and tried to pretend they were gone.
I think the best thing to do would be to 'move in" and find out what happens. Once you get passed her 'wall,' you may discover many things about her. Insecurities, pleasures, dreams, wishes, etc. and it may explain how she acted earlier.
That's enough of my opinion.
"I still find each day too short for all the thoughts I want to think, all the walks I want to take, all the books I want to read, and all the friends I want to see."
- John Burroughs