The first thing I would suggest is go to a councilor. Sometimes an outside 3rd party will be able to give you both advice and suggestions. Tell you when you're being an ass, tell her when she's being an ass.
If you don't want to go that route right away even if you do I would suggest you pick up a book by Dr. John Gottman, this guy knows what he's talking about
http://www.amazon.com/Why-Marriages-Succeed-Fail-Yours/dp/0684802414
Think of it this way, there was a reason why you married in the first place, look for it again. Sometime we make the wrong choice in who we want to spend our lives with. I would suggest exploring all the options to stay together before walking away.
Then if you can honestly say you've tried then walk...
But seriously regardless, go to the gym and put the pizza down we only get one body and if you let it go now it will be hard to get back later.