How can I forget what she did?

I was seeing this girl, I really liked her. But we weren't exclusive yet. One weekend I didn't hear from her, and then when she finally reached out. She said she had gone out with a guy she had hoped to date. He got out of the relationship he was in and they went out over the weekend. They hooked up on the second date. She said it was absolutely shitty and he was really weird and strange after. She said she didn't feel connected to him, and said cause he felt bad about leaving his girlfriend, she said he wasted his chance and she was in her head about what happened, but she was done with him.

She was very apologetic about not responding, and she tried to divert the conversation, but everything about it bothers me.

It happened a few months ago, and for a few weeks I couldn't decide whether to break things off with her or not. She wanted to get exclusive a month after it happened. But it still bothered me and I don't trust her the same way, but she doesn't want to talk about what happened and doesn't understand why it bothers me but understands it was hard for me.

What can I do?
How can I forget what she did?
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