Help! My brain is being plagued with horrible memories?

Almost a year ago, when my bf and i had broken up because he left for school across the country a lot of things happened. To make a long story short, he basically wanted his "cake and to eat it too." He led me on to believe that he was only focused on me, but later i found out that there was a girl he thought he liked. With time, i found out that they had a couple times (which is what really hurts), but I also found out the feelings he thought he had werent actual feelings. Probably lust, idk. It happened nearly 10 months ago and although ive made significant progress i still get upset about it, though rarely. And i wish I could just forget it. It was my first heart break. Any suggestions for how to deal with it? btw, he and I are currently together again because of course like clockwork, i know this week long fling was nothing. anywho, what do i do?????!!!!! im desperate!