why am i constantly so worried

for around the past year, everything has been worrying me so much, and it impacts my daily life significantly. I usually dont even have a good reason to worry and when i try to explain to someone i always feel like im being silly or insecure, this used to not even be a problem with me. A perfect example is my girlfriend, who is very loyal and loving, is going to a drinking pool party tomorrow with a looottt of strangers and alot of guys. Ive been stressing like crazy even though i have so much trust for her and i know it will be okay. how do i cope with this overwhelming feeling??? is it based on insecurity??