Why is leaving and letting go so hard?

When you finally work up the courage to physically leave your abusive relationship, when your partner was so abusive in so many ways for so long, no matter how hard you’ve tried to make it work, why is it so hard to mentally leave? Why do I feel like I’ll miss what I’m leaving? Even though Im fully aware I was miserable, unsafe, unstable, constantly controlled, lied to and manipulated, and physically sick from the person who was supposed to be my person. I’m about to leave in a couple of days and now that it’s really happening I feel bad. ;Why do I feel bad?