I (F27) found out my boyfriend (M27) had a woman sleep over our apartment

My boyfriend of 4 years (we live together) recently had a female friend sleep over on our couch after a group of them went out for the night (he slept separately in the bedroom). He didn't tell me he let her sleep over and I was out of the state visiting family. She's an old co-worker of his who no longer lives here. Years ago they slept together a few times and she was recently in town visiting friends. He told me she would be in town visiting their mutual group of friends and I let him know I was uncomfortable with him hanging out with her given their past. I also have never met her.
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Come to find out he had her sleep over after they all went out drinking (he generously offered up our couch when she shared she didn't want to sleep at a female friend's apartment due to the cat-he can never say no to anybody). I found out weeks later that he let her sleep in our apartment. He also messaged her a few weeks later out of the blue saying he had wanted to make a move on her but chose not to.
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So clearly I feel disrespected and find it all incredibly shady. He finally showed me his messages on Facebook that proved they didn't sleep together. But he still had her sleep over without even asking or telling me and sent those flirty messages afterwards. He's spent time apologizing but I can't seem to let it go. We've been together for about 4 years so it's hard to walk away. I just think if he does something like this he'll do it again in the future. What do you think?
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TL;DR: I'm livid my boyfriend had a woman sleep over when I was out of town and don't know how to handle the relationship moving forward


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