Val - I have to agree with the other guys. I know how it feels to really care about a girl, and then they up and decide they like the general attention of nobodys. Funny thing is, once you pull your head out and realize they don't deserve you and move on, then they kick themselves. At least that's what I experienced. But once I was gone, I had no desire to accept her back...too many other chicks out there who didn't try to jerk me around. Accept no bullshit my friend.
The fact that you helped her get the body she now has (and it sounds great, btw) and she isn't grateful for that, well, that tells me she neither appreciates you nor seems to give a rat's ass about your feelings or the 2 years you've both dedicated to the relationship.
If she's treating you like a dog and giving all her attention to other guys she doesn't even know...that's a big tip off. Do you think you deserve to be treated in such a way? No. And obviously she wants to keep you around for her own benefit, telling you she'll marry you tomorrow, yet treating you like ass. Don't stand for it! You're sleeping on the couch!
You have another woman who WANTS you. Maybe she's not as hot as "your girl", but from the sound of it, neither was your girl when you met her. You helped get her into shape, right? So, train this new chica and let her give you that workout she's talkin' about.
Like Cromagnum said, either your old girl will come running back because you're not giving her all this attention she's used to from you (even when she gives you none) and she'll know you're perfectly capable of being with someone else; or she'll keep acting the same way. In case of the latter, you'll know she's totally undeserving of you and you'll have someone new to play with.
Whatever you do, even if you don't hook up w/this new chick, DO NOT LET YOUR GIRL WALK OVER YOU ANY LONGER! For your own sake bro...