Mistake or Misinterpretation?

I'm in a relationship right now, and have been for about 9 months. We've been really happy, with a few bumps here and there, but nothing too serious. However, as of tonight, I recently talked to my girlfriend about something that had been disturbing me. In the beginning of the relationship, I didn't know that she wasn't single, or, at least she never clued me in on it. Now, I asked her out, and she said yes, while she was in another relationship. That day, she broke off her other relationship, which was good. But she remains friends with him, which shouldn't bug me as much as it is, but she still likes him. Now, I guess I'm just threatened by him because he still holds part of her attention. I was told that in the beginning I was a rebound, but it evolved into something more. I know that we had a really strong mutualistic relationship. Now I'm simply afraid that since he's returned to the scene he's going to get the center of attention, and ultimately my girl. Now, I guess I would be more in question of the mechanics of this type of betrayal if it had not previously happened to me four different times. I see the beginning, but it is much different than the others. There is a distance between my girlfriend and I that was not previously there when Mr. Ex was out of the picture. I've feared the change for a long time now, I just haven't given it much thought, and now that it's on the fore-front, it's obviously keeping me preoccupied. Now, I guess do I trust everything my girlfriend has said, even though I know she is capable of playing me, or do I end the relationship, and give it a quiet aquital at most?