It seems a lot of your emotional disconnect from your wife has to do with her putting on weight and losing her libido. As someone who used to be obese, lost it all, and has kept it off for years, I doubt I could have done it without my husband's love and (more importantly) unwavering support. I mean...the dude cooked low-cal meals for me. Sure, he ate most of them, but still...fewer leftovers sitting around!
And my libido went through the roof once I was okay with being nekkid.
Is it possible that your wife just needs that kind of backing? If so, bailing on her might hurt worse than it would for someone who hasn't gained that much weight.
If my husband had left me while I was still struggling, I would have believed he'd just given up on me. And that would have hurt more than cheating. Just sayin'.