Ok so my boyfriend used to sleep with one of my best friends - they only ever had sex on a few occasions over a few years and never dated or had feelings for each other ... after this he then dated his actual ex (who i dont know) ..

i bumped into him, began talking to him and fell madly in love ... year and a half later...

i keep bringing up him and my friend, asking questions and getting jealous - he told me that his best mate had slept with a girl he thought he was in love with so he had went off the rails and just used girls / sex hense how he had ended up involved with my friend.

it doesnt bother me when were all together or anything its just in my head - i picture them. i know this is silly but he said he had never had sex outside so we did it - but then six months later he admitted to having sex with my friend outside now i feel like he lied to me and can i trust him - he says in his head he didnt count it as anything because it was nothing - im just so hurt

he says me bringing it up is pushing him away - i love him and trust him completely - i know hed never cheat and my friend loves me like a sister - i'd trust both with my life - so why am i driving myself insane over this?
how can i stop this?