Sun,
I think you answered your own question in your opening statements. You say that you overeat when you are in a relationship where you feel "trapped" and you feel you have been overeating for the past several months. This sounds like a red flag in regards to your relationship. I would explore this before taking the huge step into marriage. The fact that you're also concerned that your fiance will leave you over a 15 lb weight gain makes it sound like your relationship is very fragile. I'd suggest exploring your relationship, rather than your eating, whether with or without your fiance, with a neutral third party.