Why is it my fault she had the affair? I'm supposed to forgive and move on with her?

She dated him twice before us. We are 21 years. 19 of that married. She says he isn't dependable and tempered. 2 years ago they met when she was out of town for work. I called and talked to her not knowing he was there. She said "I love you" to me. He got mad and left the next day without saying a word while she was a work. She says that was the only time and they did have sex, twice. I'm not buy it.

Why is it my fault she had the affair? I'm supposed to forgive and move on with her?
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