How does one stop overthinking their relationship?

So I'm in a tricky situation, my dad doesn't like that my boyfriend is in a different city, two hours out to be specific, and he can't bring him up without saying " you never know about someone ina different town he could be living too different lives", and what's fucked is I'm starting to overthink my boyfriend whereabouts because of my dad saying this. I trust my boyfriend, but at the same time I can't shut off my head when it starts telling me he's being suspicious and probably seeing his in town girlfriend..
How does one stop overthinking their relationship?
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