Why do I feel like this?

I have this jealousy/ worry thing and it really annoys me. My boyfriend is amazing and I truly trust him but there is something that keeps telling me I shouldnt but I keep thinking he is flirting with other girls and things like that when he has told me he doesnt. My ex gave me his password for something so I kinda read some of his emails which were between him and other girls flirting (when we were together) and I think that might be why but this may also be because my boyfriend is close friends with girls and has a picture of him and a girl laying on a bed and basically the picture of them is putting out that they could be together and I asked him about it and things but he was just telling me that he was annoying her because she fell asleep… He says that he thinks that I was being controlling and things last year with our relationship and I was overly jealous which ended up in him being at his best friends bday on the phone to me for 2 hours… and I dont want to be like that again.. I am still confused why im having such a big problem with jealousy… could someone possibly help ? =s I really dont wanna lose him over something like this