Advice please.

my Boyfriend constantly brings up the past and throws it in my face. Let’s just start with this :i was a completely different person back then (3 years ago) than I am today. I did not have a care In the world about anything besides partying. When we first started seeing each other I never thought we were gonna be serious or whatever. I was only 17 mind you. He’s 33. To make a long story short I went behind his back and had a threesome n went back to his house after. I regret it I don’t like to even talk about it. He brings it up 24/7. After I found out I was pregnant with our son cole I changed completely. I fell deeper in love with this dude and wanted a life with him. But now fast forward to today, All he does is constantly bring up that one mistake I made . I’ve been nothing but loyal to him and he does not trust me because of that one thing that happened years ago. I honestly didn’t consider us dating at that time anyways... any advice?

Edit- i agree that the age difference is a huge red flag. My mom said “ why the hell do you think he can’t get a girlfriend his own age? Nobody would put up with his shit” and that’s true shit. I shoulda known better. Now I’m conflicted on what to do. I live with him and he supports me and our son but at the same time it’s a fight 24/7. He doesn’t trust me apparently. I can’t even go hangout with my sister or go to my moms without him thinking I’m cheating. I’m 20 years old. I have no friends, I literally don’t do shit for him to be this way.