Hi and welcome Ted 🙋♀️
Being cheated on is a big deal and there can be far reaching consequences beyond the relationship itself; self esteem, confidence, trust and so on. If you haven't had complete closure on this issue, then doubt and the what-ifs will become more and more invasive as time passes.
You're really brave to reach out as you have so well done. It's hard trying to cope alone when the mind won't allow sleep or calm, so you've come to the right place. We're old hats at that game. I hope I can help in some way to lessen your burden.
Searching for answers isn't as productive as confronting the questions themselves. Something as simple as; "Why am I feeling this way?" says you ARE feeling that way and DO have doubts/fears.
A statement might be more appropriate. "I'm fearful ... will happen again and have doubts about ..." The brain will accept there's no question to answer so it begins to calm. And, you've disclosed a truth you may not have wanted to face which is scary, but an important step.
Writing these sorts of things in a journal can be really helpful. Repeating it back to yourself out loud also uses a different part of the brain. You're probably wondering why I mention the brain so much, well we spend so much time thinking it's our fault; everything! But it's not, it's the way our brains work.
When we experience a lot of stress over long periods, our brains become tired and don't work as well. Seeing your GP for something to help you sleep will rest the brain and provide a clearer mind the next day. Getting a MH Plan and referral to a psychologist will support your need to talk things out with someone who has no vested interest in your life, unlike friends/family.
I'll keep an eye out for your reply Ted. I'm open to chatting and supporting your journey.
Lovely to meet you;