How can I not let this bother me for my future?

Hi everyone,
my ex boyfriend and i always always always never seen eye to eye in this situation. he followed a lot of bikini girls on ig and recently he hung out with girls with his guy friends and never told me anything. i always had to find out on my own because his friend always posted the picture on instagram. i always felt like following bikini girls was bad and i definitely think hanging out with girls is cheating on me. i don’t want to think that way or be insecure about that for my future. is there any suggestions that i can do to not feel that way anymore? i don’t want to ruin my future relationships with this insecurity.
How can I not let this bother me for my future?
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