Hi peapod. When my (then) partner cheated on me, I wanted to know if the other woman had a flat stomach. I was obsessed with it for some reason. I was overweight and felt ugly. I asked the question, and he answered honestly, because I had asked for honesty, no matter how brutal. The answer was yes. It made me feel sick.
I hasten to add, I knew what she looked like, because I knew her. I asked the question because I was blaming myself, and needed confirmation that my weight was the reason he cheated.
It wasn't. It was just such a painful time. I asked painful questions so I could pin it all on myself. I needed more confidence in that situation, but of course, he had totally stripped any that I had away from me in his actions.
Your husband is 100% to blame for this. You are not responsible for pushing him there with your perceived looks, nor is the other woman responsible for pulling him there with hers.
The only advice I can give is to start doing things for yourself that make you feel good, without him. You won't gain strength from him, or his answers to those kinds of questions. You will gain strength from working out how to be happy on your own. This way, whether you stay with him or not, you will be safe in the knowledge that you don't need him.
Don't compare yourself to this woman anymore. You are clearly way better than that, and it's a completely undeserved torture that you are putting yourself through.