So I have this issue gentlemen. I don't know how to be an ass and not give a damn. So I need some advice.

Quick background - dated a girl for over a year (intense relationship, first love, etc.), broke up, she's now dating another guy but playing both fields and saying she's going to leave him for me (yeah I know she's not, that's why I need to get over her).

And before everyone starts making suggestions, I've already tried hooking up with another girl. Didn't work. I just felt incredibly guilty the entire time and afterwards too (I was already broken up at this point so no, I didn't cheat) and ultimately I still had feelings for her. Also, I'm stuck in a lease with her for next year so I can't just block her from my life. We're working together on a lot of stuff to do with the house so being a dick is just going to make my next year a living hell.

So help me out, because I'm sick of being led around and having my emotions dragged through the mud.

You need some serious help bro. I guess what you need is a diversion. Think of activities in which you really like doing with some other people like friends or family members. Invite friends to go clubbing and do other stuff. Keep yourself busy and try to avoid seeing her.