Why am I emotionally abusive?

I am in therapy now and I still don't know why I am the way I am. My boyfriend recently broke up with me because I was very verbally abusive to him and I would say the meanest thing in the world to him. I never meant it though. I just would say it to hurt him. Now that the relationship is over, I have so much guilt. I can get past the relationship is over because we were too different and couldn't get along, however, I can't seem to shake this guilt. I feel so worthless and deeply saddened by it. I don't know what to do or think anymore. My therapist told me that he wasn't good for me because he enabled me to do the abuse and that is more reason why the relationship wouldn't work. He would sit there and take it and not say anything. With all that said, I still can't help but to feel guilty everyday. What should I do? I know I don't want to be with him. But what can I do to finally move on? Please don't judge or say negative things, just useful advice only. Thanks