should this bother me?

I have been dating a 48 year old women for 6 months that was divorced 2 years earlier. After she was divorced see met a high school now married friend. He was supportive when she needed a friend. They went on a bike trip where they had a weekend fling. He told her he has not been intimate with his wife in years. She says they are just friends now. They text and talk. I told her it bothers me that they carry on a friend relationship and she dismissed my feelings about this at first. She said "It was just one weekend" She is normally very sensitive and considerate of others. Then she told him they need to break things off. He got mad. I later felt guilty I took a much needed friend from her and said it would be ok with me if they communicated again. His wife knows nothing about them. It still bothers me. So do I struggle with insecurities, too sensitive or what? Is she just being selfish and not very sensitive to me and his wife? Maybe he's playing her?

No, I think your feelings are justified. This is not a good situation. Someone is bound to get hurt and that is probably bothering you inside.