i need help. how do i get over a short term relationship (3 months) with a girl who dumped me for her ex boyfriend. she's engaged with him two months after our breakup.

it was destined to fail because she wasn't over her ex (didn't know at the time), i became a needy manchild, she has daddy issues and many more shit that killed attraction from both sides.

i found out i was a rebound, she probably never liked me, i keep telling myself I'm getting over it while wishing for her marriage to fail... she talked so much shit about her ex and now she's with him again.
why does it hurt if it was meant to fail?