My girlfriend went and saw her ex and had with him. How can anyone trust women anymore?

We've been together for 4 months, we started bickering but it was just over small things. Anyway, i've been kind of distant from my girlfriend from the beginning because I have been cheated on a few times and I was worried about getting too close too fast. She felt I didn't like her, despite my reurances. She's a very sweet girl and only wanted attention from me, but she was also a bit crazy, insecure, and very jealous. This past weekend she said she was going to see a friend, but because we started bickering again she went and saw her ex who since weve been dating has constantly been trying to get back with her ever since he found out she was with me. Ironic. She slept with him and when she came back she told me what happened and how she wants to be with him, because i'm so distant and she's scared and doesn't want to be alone and how he promises to marry her and have a family. She says she broke up with me so it wasn't cheating, but all I remember is that we argued. Anyway, I explained why i've been acting the way I have been, how much she means to me and how scared I am of being cheated on and lied to. After everything, I told her to decide who she wanted and she said she couldn't, that she wanted both of us, so I said i'll make it easy for you and wished her and him a good life and hung up. Did I do the right thing?