I have a problem

I got out of an abusive relationship 4 weeks ago.. My ex doesn't leave me alone and always come to my house and demands to see me ..
She as recently thought she is back in the game .. But I have slept with someone 2 weeks ago.. And Ofcoarse didn't tell her - isn't her business
Why do I always feel like I can't let go of her? And forbid her to manipulate me.. I don't want to be with her .. I am talking to other people aswel, and I know I'm so much happier without her .. When she comes over, I'm back to feeling miserable.. 
She wants to always msg me 24/7 all the time.. And if I say I'm tired and just want a night to my self she abuses me.. What can I do to be stronger ?