If you find yourself constantly worrying, is it worth it?

Alright so first let me say, i’m less asking for advice and more just wondering what different people would or have done in this situation. My boyfriend has a really bad track record with me but we have started fresh and ever since he has been literally amazing. We have been through a lot together. I really enjoy being around him, he is my best friend and I can’t imagine life without him, I love him but i’m not in love. I do have romantic feelings that are quite intense.
I find myself constantly worrying that he’s going to leave me, not his fault, I am insecure but its affecting me a lot. Everything he does I worry about who he’ll meet and what he’ll do, sometimes I think he’s just waiting for someone better to come along. I have reason to think this which is his fault, but things have changed a lot since that incident and he tells me he loves me and only wants me. I believe him, doesn’t stop the worry though. I do not want to break up with him and honestly don’t know if I have the will power to do it even if I decided to.
In a situation like this, what would you personally choose to do?