Regarding relationships

This Saturday is my girlfriend's friend's (who is a guy) birthday party, and she wants to go. However, I’m not entirely comfortable with her attending. I care about her and tend to overthink things, so I don’t want to come across as overly possessive. We've been together for six months, and we both love each other deeply, but I can't shake this uneasy feeling. Should I let her go? I’d appreciate any thoughts on this!