Why is it so hard to leave a toxic person??

I’ve been with my boyfriend for 3 years.. he’s cheated, put his hands on me, breaks up with me whenever he feels like it, disappears for days (or weeks) at a time and the list goes on. Today he broke up with me once again, blocking my number and on every social media platform.. and I’m just heartbroken, he does it all the time and it never gets easier. This being said, why is it that I can’t let go of this relationship?? 😩