Fear of my gf cheating, how do I get over it?

Hey everyone, I’m 19F and my gf is 20F, we are both bisexual. We’ve been together for 8 months and this is the best relationship of my life by far.

I love her so much and wanna spend my life with her, but for the past few days, I’ve had overwhelming amounts of fear that she’s going to cheat on me or leave me. I typically have peace of mind and don’t feel that way. She is so loyal and trustworthy, she literally would never do that to me.

I’ve been getting in my head about the fact I’m her first girlfriend, and we are both still closeted so people don’t know about us. She’s also only been with 1 guy before but he was abusive, and I’m worried that she will want to experience more stuff with other guys.

Idk how to get out of my own head. I do trust her, but it’s a fear that I just can’t control.

Any advice?