My girlfriend is close friends with someone who's bullying me...

Fully explain what he has done to you to her
You can't choose her friends but she can choose to respect you a bit more and keep your two friendships separate
Tell her how you feel- maybe explain to her that you feel like she just drops conversations when he comes and joins the group (she probably doesn't notice it). Also voice how you feel that her actions and words are contradictory

The best you can do is communicate and voice your concerns
It's up to you how much it matters that they are friends

Can I ask how he has pshychologically been bullying you? As you haven't given any examples?

thank you so much for your advice, i definitely will voice my concerns, i just wasn't sure that was the best course of action to take.

As for examples of bullying, it's really subtle things like leaving me out of conversations on purpose and talking over me when i actually do talk (i am a very anxious person so i tend to not talk in big groups), inviting everyone out in front of me except me and when someone else asks who's going he says "everyone who matters", belittling my phobia of crowds, implying I am not clever or good enough to study dentistry, whispering about me when I am there so I hear him mocking the things i say, and once he implied I should kill myself (i can't get the exact phrasing but he basically called me useless and it would be beneficial to everyone else if i wasn't here). Generally lots of little things like that, i know it doesn't seem like a big deal but when someone goes from being your best friend to doing this every day it really does grind you down, especially as he still means a lot to me but i just dislike the person he's become. it's like if he has a friend that is too boring for him (which I think I was because I don't particularly like parties) or he can't use he just cuts off from them. Basically it's a really toxic friendship that I need to escape from but I can't all the time my girlfriend is talking to him right in front of me and ignoring me.

I'd say just try and brush it off- hard I know as he was a very good friend of yours but he's not worth your time at all
I could imagine your girlfriends response would probably be to just ignore it but try and bring it up and ask her to see things from your point of view and understand that all these small things build up over time

I hope you can escape from this friendship permanently at some point

He's a jerk. It's obvious he is passive aggressively insulting you.

If your girlfriend truly thinks of you as a priority she would make herself distant from him. In terms of her birthday, it should be that you are the priority guest, not him. I think this whole thing isn't working. You've talked to her and she still does the same thing (is she attracted to him? because that's how she's acting). If I were you I'd break up and also find a completely new circle of friends.