Why don’t I have any feelings after a break up?

When we were breaking up or going back and forth about whether or not we should stay together I was sad and stressed and a big mess. but once it’s final I don’t really feel anything. I detach completely and feel like I’m over it. I wish we could be together I wish it could’ve worked out but I don’t feel sad 99% of the time. I don’t feel anything tbh. It’s been like this before with my last break up I don’t get it. I also did this with my brother, I missed and cried over him for years after we went no contact but then he came back and I learned he was racist and homophobic and mysogynistic and immediately detached and I don’t care at all anymore. Why do I detach when I know it’s final?