Why does it hurt so much to see her move on?

hi guys we met in our first year of college and fell deeply in love. now we’re in final year and she’s happy with someone else
we were together for 4 years and everything shattered 3 months ago when i found out she was talking to multiple guys late at night for hours and hiding it from me. the worst part is this was already the 4th chance i gave her. yeah i know i was crazy in love with her.
now she seems happy with a whole new crowd and people keep telling me she’s probably dating some junior now or something. honestly i don’t even know what’s true anymore 🤗
it’s been 3 months and i still can’t handle it. i’m exhausted. i feel so alone all the time. it hurts seeing how easily she moved on while i’m here barely holding myself together. i can’t do what she did i can’t just text random people and act normal after everything.
sometimes the only reason i keep going is because i think about my mom. that’s literally where my strength comes from right now. but man this pain is too much sometimes
and before anyone says go hang out with friends or keep yourself busy i tried. none of it works. i’m breaking down in the middle of crowds and every time i see her at college i feel myself falling apart again.
i’m tired man. i’m really tired.