Why does it hurt so Fucking bad?

Why do break ups hurt so bad? I can’t take the pain anymore. I lost everything this past year. My ex kicked me and my 3 kids out on my birthday exactly a month ago. Had another girl living with him days later. He’s narcissistic, abusive, mean and hateful. He seriously tortured me mentally and physically the whole relationship. Why the fuck am I still hurting? I lost everything over him. Why am I still dreaming about him? Why do I still hope I get that “I’m sorry I fucked up text” that will NEVER come? Everything about him was a red flag and toxic but to me WE were beautiful. Now he has her... and I can’t stop thinking about him...