I need to be less possessive over my bf..please help?

We keep having major fights over this. I admit I am insecure when it comes to things. But he is overly flirty to any female. I wasnt possessive until I caught him talking to an ex that he SWORE he wasnt talking to. I dont like her because she treated him like sh** and he would always be mad when he got off the phone with her. We both decided it was best to just cut her out-and he has. SInce that lie, I have been obsessing over every girl he talks to. Mostly the girls at his work, who both him and her flirt through emails and stuff. He is calling me possessive now, because I have to know who he is talking to and what is in his email etc.. I just do it because I need to gain his trust back since the ex-gf lies. I do need to chill out though cause I even get jealous of some women who are like 35/40 (hes 24) flirting with him who i know he would never leave me for. What can I do to just ease my mind? Please hlp if this has happend to you...