You don't want to beat this mistrust - it's an accurate assessment of your current situation. Your gut is pounding a drum, but you don't want to listen to it. There is no magic here, nor mystery: she's seeing another man and/or has been open to seeing other men. She just ins't mature enough to talk to you about it honestly and to tell you that she's not ready for the responsibility of a monogamous relationship. She's showing you, and you need to listen to her behavior, not the words or mixed messages.
When you don't trust an untrustworthy person and you think it's a problem that you
have, you're likely to have traits of codependency
. What would serve you much better would be to honor your gut feelings more and trust them. It's not an accident that she went on a road trip with another guy a week after you broke up. She wanted to do this with another guy and did. Sure, she may love you, but that doesn't mean she's dedicated to your relationship. Her behavior shows that she isn't ready - she sends you mixed messages because she's not settled on one side of the fence. "I love you and want to go away with another guy and here's a key to my apartment and I'll be there except for when I'm on dates and road trips."
Look at the link on Codependency above, and then look at how you can deal with core trust issues
. Trusting yourself is an important skill, and we all can learn it.
Good luck with your talk. Hang in there and beleive that you're worth the best life has to offer - you are.