I don't want to say you're cooked with your current relationship, but you might be.
For starters she doesn't share the same fetish/kink. Now that shouldn't be a problem if you're aren't fucked in the head and it gets in the way in the bedroom, but judging by what you said, you are fucked in the head and you are actually losing interest at this very moment.
Why? You are talking to other women that share this fetish. Yes you feel guilty, but you're already in the steps of falling out of love because your GF doesn't share your sexual interests. Her not liking her belly touched is already a killer.
If you already expressed about liking her body and being attracted to chubby girls for example, and that didn't help her feel good about it (I mean liking it more than she thought she would, letting you touch her belly without any problems, etc), then she won't change her mind. Reason she says she's fine with it because either she knows you like it and is just putting up with it, or too lazy to change it. You could try to talk about your kinks again with her again, but tbh this may be the beginning of the end of your relationship if you aren't sexually satisfied with her.
Living out this fetish is possible I'll say that. That don't mean your girl has to be ginormous since that will be a pain, but like, every now and then doing stuffings, bloating her belly, long as it's not overdone it can happen. This fetish is niche (in the open world) I believe so it's just trial and error to find someone who actually interested in it. Most fat women hate being fat. Hell most models are in it just to make bank.
Just to close this off and sorry for the yap btw, we aren't here to tell you what you should do with your relationship. You're a grown man and you have your own choices on if you want to keep going or not. But as stated you are falling out of love currently, with talking to other women and even questioning about your relationship here. You're doing her a disservice of not being honest with her. If you want to fix it, talk to her. If you want to leave, talk to her.