You two were dating, sort of. Now you are not. There is nothing wrong with him seeing someone else.
That being said, it does not change the fact that the man you most recently dated was manipulative and abusive who didn't care about you as much as he cared about how he could use you. It also does not change the fact that you kept going back for more abuse.
The pain you feel now because he has moved on while you want him back in spite of this abuse actually has little to do with him. It probably has a lot more to do with your co-dependency (which predates him by a long shot) more than anything else.
Please get help. Go to an al-anon meeting or join a co-dependents group and try it for awhile. You need to break out of your self-damaging cycles. They can help.
