(19/f) Still not over how my (21/m) boyfriend's ex (21/m) treated me. How do I let it go?

Me and my boyfriend started dating a year and a few months ago, everything was really cool at first and he was everything the right boyfriend could be, supportive, loving, romantic, except...he was still avid friends with his ex.

His ex (let's call him Seth) and him broke up about 6 months before we got together. They hung out all the time and would spend days and days together playing video games etc. and it made me extremely uncomfortable, especially when they would have overnight functions (I.e. trips).

I talked to my boyfriend and told him that there were going to have to be boundaries (I have a severe anxiety/depressive disorder which is mangled with trust issues) because they're exes and I know they were involved together sexually. He said it was fine and for the most part didn't fight me on it.

The one who did give me a lot of trouble was Seth. He would yell at my boyfriend for skyping me and not him, would constantly demand attention, Skype him all the time, and want to go to lunch with him every day. He also pushed my boundaries, complaining I was restricting him, driving them apart, telling me about their previous sex life (etc).

This was extremely awful for me and many times I was driven into extreme panic where I would want to hurt myself or have an anxiety attack.

One time we were all staying at my boyfriends house for a game night (including his friends and Seth) and Seth started talking, while we were all lying in bed, about their previous relationship and their embarrassing sexual encounters. I had already told Seth that because of my anxiety I don't want to hear anything about their past relationship. He obliged but apparently didn't care.

I broke down, I started sobbing and crying and screaming because I was so upset.

A couple months later Seth got mad at me because I didn't want them eating out together so much, he said I was "driving a wedge in their relationship" and that I was over controlling and hurting him. Then told my boyfriend he didn't want to be friends anymore if he was going to pick me over Seth.

It gets worse. A few months after they stop talking completely, my boyfriend goes to his friends house to grab some stuff, he gets there and Seth is making out with his friend (who is engaged) and claims that it's to make him romantically jealous.

This was extremely hurtful to me.

Even after my boyfriend has dropped him and I have cut him off he still tries to make things hard for us (trying to organize meetings with my boyfriend, taking selfies with him whenever he sees him around campus, adding him on social media even though my boyfriend has asked him not to).

How do I let this go and not let him bother me? I want to move on from this and leave him and his antics in the past but he keeps sneaking back in. He is close friends with all of my boyfriends friends so is this even possible? I just want this abuse and anxiety to end.

-TLDR; Boyfriend's abusive ex wont leave us alone, treated me like shit, keeps popping up and I just want to let everything go and not worry.