My boyfriend has depression and it starting to affect me. PLEASE HELP...?

ive been with my boyfriend for 9 months now and we have been on and off for 3 years. He has major depression and i think it is starting to effect me. hes been on anti depressants for about 4 months now such as zyban and 20mg of folixten (idk how to spell it). even though he has had depression for a long period of time now it seems like since we've been going out every time he gets down its because of me. two weeks ago on a friday night him and i were fighting and he tried to kill him self. i walked in on him with bleeding arms with more than 20 cuts on each arm. i see the cuts as hatred towards me because before that he sent me a text saying "i wont take it out on you i will take it out on me". the image is scared in my head and i can't stop thinking about it. ive started to hate my self and i always think of what a horrible person i am to make him to that to him self. i sit there and reflect on what a horrible person i am and how ive affected him. i know if i try and leave him he will kill him self. it feels like im in a relationship with my self as only his feelings are important. hes told me before im his everything and if he lost me he'd have nothing to love for and that puts an enormous amount of presure on me to not muck up and just cover up how i feel. i always put on a happy face but im broken inside. what should i do?