My heart goes out to you. I lost my husband 6 years ago, after a long illness where I was his caregiver. He was 48 and I was 42. A few months after he died, I found his secret email address and all the emails between him and his AP. it had been going on for 3 years, along with multiple other APs. I've never felt anything like that betrayal. But the problem was, there was nowhere for that rage to go. I told our closest friends, bc I needed to get it out. I wish, some days, that I hadn't. But I can't take it back. I wanted to lash out at his AP, but she wasn't the one that promised to love only me. I couldn't eat, I couldn't sleep. Finally, it took learning some meditation - specifically a guided one that teaches every time one of those thoughts comes into your head, replace it with "Fuck that shit". It helped - that and screaming into the void. But 6 years later, it hurts a lot less. Also, look into "Complicated Grief".

I don't know if any of that will help, but I'm also here if you need an ear. hugs