I’m too insecure. Want to break up

In early January, my bf left me for another girl because we were fighting almost everyday. About 2 months later, he begged me to get back with him and left her. We were only talking for a couple of months and I decided to give him another chance. Everything is great, he’s gone for basic training atm & I’ll be leaving to college 2 hours away by the end of next month. We’re both going to the same university except he will enroll to the spring semester since he won’t be able to make it back in time for the beginning of Fall semester. He’ll be back in the middle. His ex will still be in high school. She is in JROTC, the program he was in. He was telling a friend who’s also in the program that he will be visiting them after school when he gets back. I can’t help but feel insecure that he will see her there and I won’t even be in town. He tells me it was a mistake leaving me for her and that he would feel waves of sadness come over him when he was with her. However, I can’t get over the fact that he did. I feel too insecure in our relationship & I don’t want to feel this way anymore. Ig I’m not ready and haven’t moved on from the past, but I don’t want to break up with him through a letter. Especially when he’s over there using me as motivation to get it over with and come back.

Any advice?