How to let go...

I need to let go of my bf, child’s father. I’ve known for a long time he’s no good, but I have a problem letting go. Idk how. He’d be fine with breaking up because we have before, but I’m the dum one who always stops it and then stops him from going. I see how easy it is for him and I still can’t let him go. How do I let go? Why can’t I? What is my problem?