be less paranoid

i’ve been with my boyfriend for over a year now, we’ve had an almost perfect relationship and never had an argument. one problem, his ex girlfriend has come back on the scene and they’ve started talking more and more over the last couple of months. she keeps asking him to meet up and despite that she has a boyfriend shes told my boyfriend she doesnt really love him. i’m worried because they have broken up and been with other people and got back together agian a number of times. like the people you just cant get over. and i’m scared thats what him and her are. i don’t think he would ever cheat on me with a random person but i do worry he still cares for her. for a time they used to sleep together when they weren’t going out. hes four years older than me and i feel threatened by her and i don’t know what to do about it. i do trust him, and i don’t in my heart believe anything is going on but i keep dreaming about things going on between them.and its affecting us.help..