How can I learn to stop myself from self-sabotaging relationships?

Basically the title. I am 24f and never had a boyfriend, and I have recently self-sabotaged myself away from yet another relationship. I really don't understand why I do it.. i think it's just insecurity and fear of intimacy. Recently pushed away the guy I am in love with, the same guy I had been in love with 8 years ago when we went to secondary together. He's got a girlfriend now, and I'm struggling to move on. (it was also very sudden).

Any advice? I want to stop myself from ruining more relationships in the future. I wouldn't mind if anyone wanted to private message for more details.