I have to be honest -- your boyfriend has not broken your trust; just as my husband has never actually broken my trust. It is our own insecurities getting in the way and dominating our emotions. Whatever that girl told you -- ignore it. Everyone knows girls can be catty, and that's just what it sounds like she is doing. If you do trust your boyfriend, then there should be nothing that anyone else can say to change your mind. If you feel that your boyfriend has not shown any signs of infidelity (sure, maybe he should be trying harder to be understanding, but I've found men seem to struggle with empathy compared to women), then focus on the rational thoughts. Remind yourself over and over that your thoughts about cheating are irrational and only causing you unnecessary stress. Every time you start to feel the emotions, try to break the cycle -- slow down, take deep breaths, and remind yourself "These are not rational thoughts."

When I was having the same issue you were having (not letting go), I eventually realized that I was using these emotions to represent other things I was upset about in my relationship. I didn't want to deal with the new emotions, or the new issue that upset me, so I'd subconsciously revert back to these emotions and memories. Once I realized they were "placeholder" emotions (comfortable because they're known), I began to ask myself "Okay, you're not really upset about this anymore. What are you really upset about right now?" and that actually helped a lot.

If you think this relationship is worth the work, then keep going. No relationship will be free from all issues and pain. And honestly? In my own relationship, once we got past all the crap, our relationship has been much, much stronger.