poisonous relationship

I'm giving my boyfriend one last chance. He randomly disappears and leaves me in the dark for days. He will make promises to me that he never keeps. He goes behind my back and constantly lies to me until I catch him on the lie. If I ever tell him I have an issue that needs to be resolved he will just get defensive and somehow push all blame on me and say he's just stressed out. He will (I believe jokingly) punch a wall right next to my head to possibly scare me. 
I have been nonstop there for him. The second he needs something I am there doing it. I give him my all and care so deeply for him, but it's not mutual in the slightest. 
I wrote him a letter and left it at his house last night, so he should eventually see that. It told him he had one last chance before I go. In addition I stated that If he wanted to be a part of his kids life then he needs to clean up his act this second. (I'm currently 9w) 
Please offer up any advice you have. I need all the help I can.