Why am I a jealous person?

So im 18F and my bf of a year now is 19M from even before we got together (we were friends first) I noticed whenever he talked to other women I got really jealous. I have told him about this its something he's just accepted but its getting irritating for me because sometimes it affects my anxiety to and most of the time it is extremely irrational like we were hanging out with his friend and gf and I got jealous at that which is ridiculous. We've both had one relationship in the past him he had some issues when it came to other guys so his fears are reasonable especially at the start. but I never had any possible cheating or anything with my ex so idk why im like this if someone has any idea id love to know.