Getting over an insecurity of your man going out

How do you get over an insecurity about your man going out if you ever had one. My man turned 21 today and he went out to celebrate his birthday basically all day and night. I cannot go with him because I am only 20. I will be 21 soon. How do i get over me feeling this anxiety about him messing up. I feel like he is going to find more attractive than me and want to get their numbers and stuff. Makes me feel so insecure. How did you let your mind pass all of this.