Losing weight to keep relationship going.

Both my Fiance' and myself are 20 years old this year, we've been dating for four years and engaged for one. Since we started dating back in high school we have had many struggles, (getting through high school mainly) (smile) one of them being my weight gain. I was heavy when we started dating but I've gained probably 90+ pounds since then. It seems that over the past year, he has grown to dislike my body and when we have sex we end up role playing where I am much lighter and much more erotic. He's also been having a lot more fantasies about other women; which I completely understand if he has lost interest in bigger women. To add to this, he has cheated on me in the past (both real life and cyber) and there have been many trust issues but we always seem to make it work out. We realize that we are unlike most people at our age because we are both willing to do whatever it takes to make our relationship work; we truly do love each other.

I do not feel that I should be chastising him for not loving my body since I have gained a lot of weight, but I can't help but feel alienated whenever he is around other females. I'm working very hard on my weight loss but I'm looking for some advice on how to deal with the feelings that I get whenever I look at another woman and realize that my Fiance would be much more satisfied with me in bed if I had her body.

Please understand that we are not breaking up because he cheated; we are far past that, and we are not breaking up because we are incompatible sexually-we just want to work this out.