I (19F) constantly compare myself to other people and I feel like it will ruin my relationship with my bf(20M)

So I’m not really sure how to start this post since i’m new to reddit but i really need some help. I’ve been with my bf (20M) for about 5 months now and as more time passes i’ve noticed i’ve become more and more obsessed with how he perceives other women and if he’d leave me for them. I understand that can never be a good foundation for a relationship and i’ve tried to fix it but i can’t seem to get out of that constant loop.

What drove me to write this post is that tomorrow my friend’s throwing a birthday party and this extremely pretty girl(we’ll call her Mia) will maybe be there. We went to high school together and i’ve been compared to her numerous times. Basically whenever that friend whose birthday it is compliments me (and she does pretty frequently) she always mentions I am soo pretty but not as pretty as Mia. She always ranks me and has said multiple times how we shouldn’t invite her if there’s guys coming cause they would like her. This has really stuck with me and she basically became an obsession for me and I constantly compare myself to her. What’s even is that people always mistook us for each other and said we look alike but i just don’t see it and feel less than. I get that she’s attractive and that I’m attractive but I just can’t get the thought that, when my bf sees her he’ll realize how prettier she is, out of my head.

I’m sorry if this sounds stupid but it’s been bothering me to do point i’m dreaming scenarios and i can’t calm down. I’m scared that if she comes i won’t be able to enjoy the party and potentially ruin it for me and my bf.

So what i’m asking is, how do i fix this insecurity and completely understand that just because she’s pretty doesn’t take away from mine or anyone else’s beauty? Any advice is appreciated.