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.
LemonVibe 
!. “Fought to be with”? Fought with whom? (you don’t need to answer me – this is for you to consider) This makes me think that although you may be on board with this relationship it has a rocky foundation and has made you feel insecure.
“best friends with someone she’s known for almost 6 months, who she knows is in love with her?” Yes, it is ok for her to be best friends if that’s all they are. It’s not her fault that the person has feelings for her. I had lots of friends who were in love with me in high school I had no idea about and lots when I played video games I did know about. I was honest and up front with them that I was in a committed relationship. Some took it fine, we’re still friends and they moved on with their romantic life. Some did not and I moved on. That’s part of being a adult.
“It has been confirmed by mutual friends, that were confided in by said ‘best friend.’” You are definitely on a rocky foundation if you are telling other people your relationship’s issues. This should be between you and your girlfriend or at least you, her and a counselor or couples therapist. It’s NO ONE ELSE’S business.
“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?” Depends. That could be cheating, or that could be work, or just a regular friendship. You should discuss it with her. Use the words “I feel” when you do and if you start getting angry then you may need a mediator.
“I am told over and over again my girlfriend has zero feelings for anyone other than me.” If that’s the message you’re getting from her, and everyone else you have now involved, then I would count it as true. If it is true I would suggest counseling for yourself and girlfriend to help you move on from your insecurities since you have issues with communication.
Best of luck to you.