Why can’t I let go of the past?

my fiancé and i have been together for 8 years but i can’t let go of the past that we’ve been through. he was talking to this girl two years into us dating. he also DMed girls on instagram complimenting them on how pretty they are. i asked him why he did it and he said he wasn’t sure of us. okay? so how come he didn’t break up with me? bc he didn’t want to he said. it drives me crazy and i can’t stop thinking about it. i know he loves me now and would never do the things he did in the past now. I don't know what to do
Why can’t I let go of the past?
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