Need help

Sooo in my previous relationship. I was just hurt very badly. He told me to trust him and let him in.. and he even told me that I could tell him what hurts me so he doesn’t do it and he used all of it against me. He used my past with abuse against me and said it was my fault blah blah blah. He also cheated multiples times and was cheating the WHOLE time which was a year. So there’s that. Very bad relationship... and nowww there’s this guy that I’ve dated and been friends with for 5 going on 6 years in 2 weeks... we dated when I was 11 and he was 12 lol. So we were young obviously it didn’t work haha but we stayed friends. I am now 16 and he is 17. I wanna give it another shot and so does he because.. I mean we’re older now. We know what to do and wtf not to do. But I have a few problems. 1. I act as if I don’t care. Whenever he says “you’re single” or whenever he says “this girl is tryna hmu” I just like don’t care.. and I say you do you on that.. or I’ll say fine, if you don’t wanna be with me then don’t that’s all you. Iike just acting as if I don’t care. And I dooo omggg I do. So much. (That’s why I’m scared). And 2nd he has a girl bsf. That he will and does choose over me. Like no questions asked. They’ve never dated and probably never will but they are still close asf. Like I know if it came down to saving me or her.. he’d save her for sure. And you know I’ve been cheated on so obviously I’m scared.. but like at the same time I tell myself “it is what it is.. if he’s gonna cheat he’s gonna cheat” which is true.

I guess what I need help with, is.. how do I show him I care without him knowing he could have anything from me. Without him thinking “damn I got this girl wrapped around my finger” because once guys get that mindset 🙄it’s hell. And also. I know deep down I’m not okay with the girl bsf thing because I strongly believe once you’re in a relationship, having those friends are fine but your girl/guy should come first and be your bestfriend. And I know the chances of that are slim. So how do I become genuinely okay with that?