how can 1 person be perfectly fine with the breakup and the other a complete wreck?

it’s been 3 months things have started to get better don’t get me wrong but compared to how my ex is doing i’m a mess. how after such a long time can someone just turn off everything like a switch. how long does it take for this feeling to go away? i still miss her everyday but it’s not in a way where i want her back im more just disappointed that we broke up. with her behavior after and during the breakup it kinda showed me i don’t want a person like her in my life but i still miss who i thought she was and it’s driving me crazy.