How do you get out of a bad relationship?

i feel like im constantly being lied too, but being the type of person I am I hold everything in, until eventually i know I will explode, i go through horrible mood swings i think its because im stressing alot. I dont know how to address the issue this person is such a good manipulator, I tried to just break it off but we always seem to be together again , i think i've lost my identity.