When you realize they're not the person you thought they were: how do you handle that?

In my case: the guy I liked was super smart and ambitious, but later on I found out that he's arrogant, selfish, emotionally immature and even a little sexist.

I feel super let down and I know it shouldn't bother me this much, but it does :\
When you realize they're not the person you thought they were: how do you handle that?
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