Hi and welcome,
Somewhere in those ten years did you have a conversation on exclusivity? Because clearly you feel cheated on ....and from the story it sounds like it might have been a few times.
Issue 1 might be the cheating and trust issue.
Issue 2 ..
Your body is telling you in many ways it doesn't want this ....sleep, appetite, kicked in the stomach feeling, mr happy not getting aroused....maybe you'd be happier finding someone else. How many months or years do you want to live like this?
The other guy is cheating on his wife ...she's been cheating on you Nice couple ....what happens when that guys wife catches him like you caught her ...how does that fuck things up ? What retirement ...now he's free to spend more time with her as much as their NRE heads can handle ....and he needs a place to stay ....wife kicked out.