Mental abuse

Tell me why people will come in ur life just to say anything possible to get u to stay and keep it going for months just so they can get what they want and not even care how many times they hurt you and in the end not give a fuck about you or what they did how does someone mentally abuse you so much and in the end act like it’s nothing?? I was being mentally abused for months but I wanted what I thought was there love when really they just wanted one thing and then gets angry at me bc I’m done and I’ve been so depressed lately bc all I wanted was someone to be there with me