My Girlfriend Is Having An Affair With Her Cousin

I have always felt my girlfriend and her cousin had a thing in the past just from the way they talked to each other over the phone and how the conversations were very awkward and short when I was around. I raised this issue with her the first time it made me uncomfortable and she told me off, and questioned how could I even think of her stooping that low with a cousin. In between, we have been going through a lot of ups and downs, mostly because she would appear to be flirting with other guys and when I ask I was told either its just another old friend or relative.

We talked about it, told her I wasn’t happy and she stopped talking to most of them (well, at least not in my face anymore). She had to confess so many other things because I had found out the truth by myself. In the process of rebuilding my trust in her I asked her one last time, and this time she admitted that yes she had sex with the cousin for sometime, what I suspected all along (maybe she thought I already knew by this time).

I was happy she opened up to me because I don’t care about what she did in the past. Because I wasn’t comfortable with the fact that I knew they were constantly in touch with the cousin (not sure about what since most of it happened behind my back), but I wanted to give her a chance because of how much she had opened up, I asked my girlfriend to text her cousin that she had told me everything because I didn’t want the cousin to ever talk to me again as I felt he was taking me for a fool.

The cousin called back immediately, and girlfriend asked me if she could take the call in privacy, and I lost all my strength and chose not to fight and said OK.

She went away but then I made up my mind that whatever she has with the cousin was clearly more important to preserve than what we had, that this conversation couldn’t happen in my ears. What’s there still to hide??? Am I overacting (given she finally confided in me)?