Have you read any posts that mention the infamous Cheater's Handbook? Cheaters often rewrite marital history and say they haven't been happy for a long time. He had other options, such as speak to you about it, suggest MC, D you first. He decided to have an A, which is not a good coping skill.
His A really isn't about your autoimmune disease - it's really about his selfishness. He doesn't want to think that any of this could have been because of his issues, so it's easier to blame you, blame the dog, blame the wind, etc.
His A doesn't really reflect on your looks. Look at all of the "gorgeous" actresses and singers who have had cheating spouses. Halle Berry, Jennifer Aniston, Shakira, and the list goes on. Adam Levine cheated on his wife, who was a Victoria's Secret underwear model. Kinda hard to compete with that, and yet their WS cheated. I get it though, my self-esteem was in the toilet for a while after.
Chaos, another SI member, has the BASGU [bad a** sparkly goddess unicorn] method. Go out and buy you some outfits that make you feel fabulous. Chaos buys sparkly undies and wears them just for herself. It doesn't have to be anything expensive, but something that makes you feel like a BASGU. (It really does help.)
The rug WAS pulled out from under you, and the overthinking part is your brain on trauma. Have you thought about IC with a trauma-informed therapist? I found mine to be very helpful during my healing journey.
BW M 34years, Dday 1: March 2018, Dday 2: August 2019, D final 2/25/21