how do i stop being so overwhelmingly insecure in my relationship

I’m a 20 year old girl in my first ever relationship with a boy, and he’s absolutely wonderful. we’re just under a month in, but I’ve known him for several years and have been speaking “romantically” for a few months now. however, he goes to another university a few hours from me so we only see each other every month or so, but talk every day. he has done absolutely nothing to deserve the constant state of worry i’m in, in fact, he’s someone that i have grown to trust wholeheartedly.

but I can’t seem to shake this awful feeling that he’s going to meet another girl and realize that he could do so much better than me. my doubt does not come from anything he’s done, but instead the fact that I’ve always struggled with self esteem. i don’t feel there’s anything particularly special or good about me. this thought process has started to take over my life, I feel anxious throughout the day and struggle to focus in my classes because of it. I’ve been on anxiety medication since I was about 12 and am considering having the dosage upped because I just can’t deal with this.

I haven’t brought any of this up to him, I’m afraid he’d think I was being overbearing or attempting to tell him he can’t be around other girls. that is NOT the case at all. i am well aware that i am creating the problem here but I can’t seem to stop. all I can think about is the fact that he could do better than me, should do better than me, but then the thought of us breaking up and him no longer being a part of my life makes me ill. i care for him so much, i want this to be good and ive wanted this with him for so long, but i don’t know how to stop thinking this way.

every time he takes longer to respond, or his tone seems a little off over text, my mind just goes: this is it: , he’s done, and he’s found someone else. and i feel so guilty for thinking of him this way.