Jealousy problems with my girlfriends guy best friend?

Hey, I'm 15, my girlfriend is 14. We've been going out for 15 months yesterday.. We're long distance so it makes things kinda hard..(btw please don't say long distance relationships never work cause that's not true) Her best friend is a guy and it makes me a little insecure.. He has made moves on her before in the past while we were together like once he kissed her cheek.. And he hugged her from behind a couple times.. You know just like boyfriend stuff.. And actually he recently told her he was about to ask her out( he also has asked her out before but she REJECTED him) but I got to her first.. And his mom told my girlfriends mom he still likes her?? And his relationship status on facebook is "Its complicated".. Like is that status bout my girlfriend? It's just really weird and she sees him a lot since they both do gymnastics which makes me feel worse.. It makes me depressed a lot and I'm always depressed whenever I hear they're texting, talking, or hanging out and I kow it's not fair to her.. I need some advice on how to cope with this.. Thanks for taking the time to read this. Any advice is useful. Thanks :) Also, I don't think cheating is the case.. Like every night before bed, we probably spend about 10 minutes on the phone telling each other how much we love them and would never wanna hurt the other and never would and we tell each other all these things like that we'll be together forever and stuff.. Anyone with a heart couldn't do that every night and then go cheat on em like that with no guilt whatsoever.. And also, she's not discreet about her hanging out with him either which also makes me believe she's being loyal.. I just wanna learn how to COPE with this.. Obviously, it's him thats the problem and can't let go of her.. Hell, I wouldn't let go of her either.. She's a really great girlfriend though and she goes out of her way so much to just talk to me.. Cheating isn't the case here.