When people break up they are free to get together with other people and they are under no obligation to tell their exes that. She is free to date whoever. 5 months after break up isn't that unusual. You act as she cheats on you. It's not called lying or cheating, it's called moving on.

I would recommend therapy as other poster and I think you should not go see her. Meet your friend somewhere else. If a break up happened a year ago it's time to start healing

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