Allowed what? You did not allow someone to treat you poorly. If they had asked for permission before hurting you, I doubt you would have said yes. The only thing you allowed was their presence in your life. What happened after that is on them.
Why didn’t you cut them off? I assume it’s the same reason most of us get caught up in the webs of people that hurt us: Intermittent reinforcement. They’ll go days showering you with affection and attention then pull it away. All the while knowing what they’re doing is messing with your head. This behavior continues for so long it results in a trauma bond. How long it takes for us to finally break free has a lot to do with (among other things) previous trauma, our attachment style, and whether or not we have a solid support system.
Understand something: You are not obligated to forgive him or let go of your resentment for how he treated you. If you can navigate those feelings in a way that doesn’t re-traumatize you or keep you stuck in a loop, then fuck that guy. If you’re like me, you’ll hate for so long you’ll become de-sensitized by their existence.