Calling her an awful name and saying you regret marrying her over something so minor? You sound like a real catch, yourself. Probably do and say asshole things to make her so insecure.

Sorry to break it to you OP, but all women are varying degrees of crazy. They range from slightly neurotic to full blown psycho. You just have to figure out the level of crazy you are willing to deal with for the rest of your life.

There are indeed a lot of studies that would support her reasoning for believing this. Still, she should have a little more faith in you. Perhaps you could invite her to work out with you?

It's pretty common my ex fiancé accused me of cheating when I put make up on or worked out

At least your wife noticed. I have lost over 5% body fat, almost 3 inches around my waist and significantly bulked up muscle and mine doesn't seem to have.

Any change of behaviour / habit can look suspicious. Just give her some time, she will realised by herself you're not cheating on her

That's a good thing, that means you're making good gains.

my father's worked out for the last 30 years and happily married... maybe she should go with you.

Sounds like the time I tried to dress up for my bf. I told him I wanted to look good for him and his response was 'you just wanted to show up other people'. (Not in a yeah you are hot voice but a sarcastic one)

Sadly, OP, that's considered a sign of cheating. Cosmo tells them so.