It sounds to me like your wife is depressed. And her relationship with you has become a cycle that feeds into the depression. She doesn't feel sexy because she's fat, she doesn't want to get naked and be intimate because she doesn't feel sexy, she wants your love and approval but feels ashamed, she wants to go out and do things but doesn't have the energy and doesn't want anyone to see how big she is... and through it all the two of you continue to grow apart. I am familiar with this.
I also gained a lot of weight after I got married, and lost it, then gained it again. My husband and I didn't have sex for over two years at the end of our marriage (we're divorcing now). Fortunately for me, he did not cheat. This I know for sure. It would have killed me. My husband tried everything you did to be "supportive" (he's very experienced in weight-training and worked professionally in a fitness-related field) and it only made me feel that he was turned off by my body. But I know that I was unhappy, which made me overeat, and then feeling rejected and ugly made me unhappy, and underneath it all was a low-grade depression being overlooked.
Originally Posted by WrongWay
. . . it would seem, I am screwed.
Your comment is telling. I think your wife is screwed more than anyone, but I could be biased. It's as if you see yourself as apart from both of them, like in opposition rather than intertwined and very much part of what's going on here.
Your wife did not gain weight without your having been complicit in some way. And your mistress did not break off with her partner and focus on you without being encouraged somehow by you. All on a subconscious level, mind you. You have a responsibility in how it all played out, and how it will play out now.
The honorable thing to do is to set aside your mistress and repair your relationship with your wife. If your mistress is meant to be in your life, she will wait. If it really is love and not just NRE (New Relationship Energy), it will survive a hiatus. It is your wife you made vows and promises to, and she is hurting. She may need the wake-up call, like I did, that comes from hearing her husband say, "I can't go on like this." Now, repairing your relationship doesn't necessarily mean staying together, but it does mean getting real, getting honest, and communicating. Because whatever contribution you made to the marriage falling apart the way it is, will be something you take into the next relationship, if you move on, so might as well get clear and learn from it. You are doing her (and you) a disservice by not coming clean about your dissatisfaction and your affair. Get into couples therapy, and work on it. Whether you find out you want to stay together or split up, stagnating in the muck and mire of unhappiness and frustration does neither of you any good.
You don't want to leave the marriage without knowing that you did everything possible to fix it. My husband was unhappy for three years and then finally felt he had to leave. I told him, that was three years we could have worked on it and come to some resolve together. I would have been much more accepting and happy - yes, happy - if we had both decided to end the marriage through mutual agreement after much talk and work on ourselves. But he had kept his feelings to himself for so long, that when he finally told me, he had no steam left to look at or try fixing anything, and "had to" move out. I was a crumpled heap on the floor for four straight months after that. Almost a year later, I am still in somewhat of a shock. Believe me, you don't want to do that to her.
I hope this helps in some way.