“I’m threatened by my girlfriend’s best friend”

Is it okay if my girlfriend, that I’ve known and fought to be with for almost 2 years, is best friends with someone she’s known for almost 6 months, who she knows is in love with her? It has been confirmed by mutual friends, that were confided in by said ‘best friend.’

Am I overreacting and being a ‘possessive dick’ when I get upset about all the time they spent together, ¬†and out-of-town overnight trips they take together? I am told over and over again my girlfriend has zero feelings for anyone other than me. But I can’t help but feel threatened by it.