Whoa buddy.
I made this same mistake myself a couple months ago. I still catch crap for it. Be prepared to catch crap with it for a LONG time if you intend to stay with this girl.
Everyone else has given great advice, which I totally agree with. Except the weightwatchers membership thing. (although if you decide to do that, can I have your guns? PM me for my contact info).
If a girl stops taking care of herself, I take that as a personal insult because she doesn't care enough about our relationship/me to do so.
However you need to keep in mind that if you DO intend to stay with her, she needs to be aware that you are attracted to her for her personality AS WELL AS her looks. Think about it this way: If you started gaining a lot of weight, stopped bathing, etc; would you blame her for being less attracted to you??
-Will
PS: dead serious about the guns.