i need girly advice.

i have a friends with benefit. and we’ve literally been seeing eachother for 9 months straight. and when i say ‘straight’ i literally mean we have been seeing eachother nearly every single day for 9 months. we’ve done everything apart from have sex which i don’t mind as i wanna wait for the right time and im not fussed anyways. but we’re technically not official, we don’t wanna be in a ‘relationship’ with eachother because we both feel like if something goes wrong it’s gonna mess up how close we are and how close he is with my family especially. so we decided to stay friends but still have that bond hence why we’re friends with benefits. But today,, my bestfriend saw him put his arm around another girl ( who he has been friends with for a very long time and it’s his ex’s bestfriend so i genuinely think i’m going insane ). I don’t think i should be worrying, but i wanted other girls advice. i’m a huge over thinker and i honestly worry at everything, i can’t talk to him about it because then he will definitely think i’m going insane because his snap is dry af lol and he lets me go on it all the time. i just need some reassurance from girls to tell me i’m just being paranoid🙁