How do I deal?

Long story short. I am back with an ex. A man I was with for 4 years. Our relationship took a left and he kicked me out knowing I had no where to go. I chose to stay with my friends for awhile and we all ended up partying a lil too much and I had a one night stand. Well, weeks go by and these “friends” turned out to be slimy snakes. I go back to my ex’s because he knew about the situation and wanted to help. He knew he fucked up from kicking me out, but now he knows I messed with another man. I love this boy and I don’t know how to go about the situation or if I should consider that he’s not ever gonna get over it??? He says he can’t touch me without thinking about it. Even tho it happen when we weren’t together..... idk what to say during these times.