I fear that my girlfriend is going to leave me.

I am very aware that this is a classic story, that just doesn't help me feel better. I am very young, let's say a teen, hopefully that won't result in deletion of this post. My girlfriend is a teen as well. We go to the same high school, and are dating for around 4 months. She is my first GF, just like I'm her first BF. I have become increasingly aware that I am anxious about her leaving me, thinking I'm weak/dumb/pathetic/ugly, or losing interest in me. She is semi-avoidant so that doesn't help either. I know I am supposed to deal with my anxiety alone, that I shouldn't expect her to reassure me, but now it's very hard for me.

A month ago we had a huge fight because I was too much to deal with. I constantly needed reassurance that she won't leave. She never gave any signs that she would, but I notice even the smallest details in her behavior, especially when she's more quiet than usual.

For that reason I try to give her space as much as I can when it comes to this topic, but today she was off. We saw each other very little, she sent fewer tiktoks, she even didn't respond to my good night text.

This sounds very stupid when i write it down, almost as I am asking for a problem, instead of already having one. But please, I don't know how to control this, it's always like that.
I try to explain to myself that she is here, that she won't leave, but I'm terrified of losing her. She was my first love, my first everything, and because I know that first love has a rare chance of survival, I constantly think about how this may end and how I would manage to survive that.

Even though today she doesn't seem a lot off to you guys, she did to me. I spent my whole day battling with myself whether she is leaving or not, whether I am boring her or not, whether she is hurt or sad, etc. How do i stop this? How do i deal with this?

How do I become the best boyfriend that doesn't need reassurance and that isn't clingy?