Wallflower in need of guidance!

Need some advice guys. So I'm really not good at making friends in a group. I tend to shy away from and become a wallflower. And it takes a long while before I can be comfortable being myself around people. (Lots of trust issues and traumatic past.) I have mental health issues: depression, anxiety, PTSD.

I'm in a relationship with my boyfriend and he sometimes invites me to play video games with his girl/friend and her friends. And I get left feeling like the third wheel. Me and this chick don't talk and she just has conversations with my bf while I'm playing the game. I never know what to say and she hasn't tried to make a convo with me...my bf knows how shy I can be so he doesn't get upset. He just says to talk if I feel confortable.

Thing is: I don't want to be a wallflower anymore. I'm HIS girlfriend. I should be more social and be able to make friends as easily as he can. My only problem is this shy nature is who I've always been and I dont know how to break it?

Guys please help me? How do you over come being shy in a big group? How do you guys not care about what your SO's friends think of you and be who you really are? I'm only comfortable being truly myself around my family and my bf. In my head, I don't think anyone would like me for who I am and will say that I'm weird, or annoying.

Honestly, I know my problem is that I care too much about what people think. I want to fit in. I've always been the "odd one". When I want to be the fun and outgoing person I know I can be. Please help me...