If it's some random girl that I'm seeing for 2-3 months and shit ends, I wouldn't care at all, heck I've had small things like that in the past few years. It just didn't work out and you move on, heck I'd bang her friends and not think twice about it. It would take me days to if not hours to forget about it.
However with my ex, we've been together for a long time, and been a part for a long time as well now. We were friends, got closer, then hooked up. Was head over heels for her after a certain point, I'd write her poems (go 'head punks laugh, sex was extra amazing if she liked them), knew her family (single dad, mom passed away). There was a lot of emotions in this relationship, and
when I noticed how emotionally invested I was into her it scared me and I ended things. Was my biggest regret, now she hates my guts


It's life, ya move on.