How do I [21M] get over the fact that I'll really never see her [22/F] again?

I met my ex right before we started college, and were on/off for 3 years until she broke up with me last year and started dating her friend's brother a few days later. I was crushed because she left me the first time to pursue another dude too..

Anyway, we don't live too close to each other, and I just graduated from college. I didn't see her today, and was really nervous about running into her. Now I'm wondering if I should have seen her/said something to her? I have her blocked everywhere and don't stalk her social media, but I do check my emails hoping she'd send me something. I want her to come back again even though I know it'd never work.

I just feel so miserable now. I don't want to be her friend, but she was really important to me, and never seeing her again makes me sad. I'm also afraid I won't have a chance to meet people anymore because I'm not in school.

tl;dr: Graduated, and thinking what this would be like if I could share it with my ex. Trying to cope with the fact that I'll never see her again. Feeling alone, afraid, and depressed.