How do you stop being gullible in a relationship?

Personally i can be very gullible when it comes to someone I care about, she can tell me the president is outside and I'd believe it, id run out there to find that mf ha, how can someone stop being gullible in a relationship cause I know outside of a relationship I'm completely mistrustful which I also have to work on lol
How do you stop being gullible in a relationship?
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