Am I vain?

I'm 2 months post partum and I've been having a lot of issues with my body image. A few years ago, I was in great shape and felt so confident in myself! I've been with my fiance for 3 years and since then, I have put on a lot of weight and I feel so gross. I really want to change my eating habits and start working out. My fiance is so insecure by this! He thinks I'm wanting to workout just to get a hot body and go look for someone else. He thinks I'm vain and that I shouldn't be insecure when he tells me I'm beautiful all the time. I try to explain that it's not about him at all, and he just can't be supportive.

*my fiance is over weight as well, but makes it seem like he's only ok if we workout together but he has no desire to.

I don't know what to do. Do I just be unhappy in myself to make sure he's comfortable? That sure as hell doesnt seem fair to me. I really need some advise.