Jealous over his co-worker

[tigerfan22]

I've been with my boyfriend for nearly one year now and we're doing great. We have both made a lot of improvement on our jealous ways and insecurities. But there is one woman from his past that I cannot (for the life of me) get over. About 3 months into the relationship he told me that there was a girl he worked with that he had been "involved" with up until a couple of weeks before we got together. Of course this came as a shock to me because he had seemed so open and honest about everything. As soon as he could tell i was bothered by this, he assured me that everything was completely broken off and they rarely talked. About a month or so after, I find out they still talked, on a daily basis. Being a computer technician he had to work on her computer whenever she needed him. I found calls with her in his work phone that last 20 minutes, longer than just a quick work question, that he could not justify. Sure, if you play with fire, you're going to get burned. But I think my snooping brought something very important to my attention. I talked (fought more or less) about it with him, and he claims he told her to back off because she was causing problems. He says he only started anything with her because he was lonely and she was throwing herself on him, buying him things, etc... He says she's everything he does not like in a girl and that I am a blessing to him. For the past few months I have done SO GOOD about just blocking their few-month-sexscapade out of my head but the second i think about it, it tears me apart and turns me into some psycho girlfriend that I absolutely hate. Why can I not get over this and accept that she is part of his past and that I am his future??