Question

I’m 17 and in Highschool (grade 12) and I’ve been friends with a girl I liked for a long time and I already knew we couldn’t be together which still really bothers me and we recently stopped talking which is REALLY bothering me mentally and today I just saw her with her ex and of course it made me feel so shitty I’m not tryna come off as this “who’s this thirsty ass teenager tweaking over a girl” or nothing like that I’m just hurting a lot mentally and could use some advice on what to do to help fix this pain