I'm going to basically agree with the rest of the thread here. It sounds like the girl you're currently with is the obvious pick here. However, I completely understand how addictive of a drug lust is. I was fortunate enough in my previous relationship to not really have anyone coming onto me during it, and her and I mutually agreed to keep things monogamous. But there are some women I've known over the years where if they were coming on to me during that time period, it would have been difficult to say no in practice. I'm talking the sorts of BBWs/SSBBWs with tits easily bigger than my head and/or asses that you could fit a tray on. Women where, in hindsight, the bulk of my interest in connecting with them came from their sheer physical stature. My ex was by no means unattractive in my eyes, but some women simply mogged her in that department.
But to my original point, a lot of the women that were more attractive in my eyes didn't have the degree of emotional chemistry that I had in my relationship, and that's kind of where our respective situations start to intersect. While I COULD have built up a strong emotional relationship, it was by no means guaranteed, no different from your situation. But that's the key word here: Could. There's no guarantee that such a bond could even form, while the bound you have with your current girl already exists. While it sounds like she will never reach your ideal physical form, the opposite could occur with the girl you're lusting for. If she doesn't end up doing something to her body to make her less attractive to you, e.g. WLS, then age will inevitable catch up to her, and the physical aspect will end up falling through regardless.
A strong emotional bond is something that's much sturdier than physical chemistry, and it sounds like you've already found that. The fact that she lets you fool around with other women is a bonus. But if the fat girl is unintentionally sabotaging the bond you already have, you may just have to cut her loose, or at the very least openly establish boundaries. Otherwise, you're just turning into the male equivalent of the woman who dumps her stable, middle-class partner because she thinks she can do better, and ends up with nothing.