Hello and thank you for your question.
I can understand your feelings of insecurity as your relationship started while she was still with someone else. Do you know if her previous relationship had been rocky or was she unhappy for some time with this other person? Did she actually cheat on her girlfriend or just start to get closer with you, and then broke up with her?
I know that she was unhappy in her previous relationship, her and her girlfriend would argue all the time. Me and my girlfriend go closer as time went by and realized we had a connection and then when her girlfriend at the time went on that trip we ended up sleeping with each other.
Okay, so of course she should have ended things with the previous girlfriend before getting intimate with you. However she did immediately break it off with her, once she returned from the trip. So, she did make a mistake but didn't continue to deceive her as well. What you will need to do, is give your girlfriend the benefit of the doubt over this. Plus the fact that you know her so well should help you to know her character. Other than this incident, is she trustworthy overall? Does she seem like someone who knows how to be committed and sincere? When your doubts come up over trust issues, you can do this. Its called a thought log:
1. what is the thought or feeling, example. she is going to cheat on me.
2. what triggered it? she did this to other girlfriend.
3. what is the evidence? here is the key. there is not solid evidence that she is being deceitful in your relationship. That is what you need to focus on, is what is happening in the present time in this present relationship.
If you find that evidence does come up that indicates she can't be trusted, then you will need to talk with her about it, and then take appropriate actions at that point.
If you feel that you cannot get over what happened with this other girlfriend, then I would really suggest you and your gf go to a few couples counseling sessions to clear up this matter, so it doesn't follow you into this relationship and you can just enjoy being with one another. Please click ACCEPT so I'm credited for my help today. Feel free to continue the discussion, even after clicking ACCEPT Thank you.