Just so you know I have been "lurking" on these boards for the last year and a half. At some point, I was going to post my sitch here and introduce myself to all these wonderful people who I feel I have gotten to know over time. I will still do that eventually, but when I read your original post, I had to get off my a$$ and register, so I could stop you from making the worst mistake of your adult life.
*DO* *NOT* *MARRY* *THIS* *GIRL*.
I was in exactly the same place you are now, only I married her and had 2 kids. Now I'm completely trapped for at least the next 15 years in a passion-less, love free marriage. I love my kids like nobody's business and I would not trade them in for anything. But I wouldn't wish what I'm going through on anybody.
DON'T MAKE THE SAME MISTAKE I DID.
As much as you may not believe it's possible, trust me, it will get much, much worse than it is now.
Good luck, and I hope you will be able to find the happiness that has been denied to me.