How do i help someone with depression?

ell.. after being together for a year with my bf. i couldnt stand it any longer because he always gets angry at the tiniest things and shouts abusive wordswhenever we quarrel. he's the most loving, most caring person but when hes angry its a different story. anyway, i broke it off with him then, and now he's gone to see the psyc, they said hes got post traumatic stress disorder. when he was 18, he got into a huge fight n got his teeth knocked out. apparently, the psyc said that he has been blocking that part out and replacing it with being cold towards people. anyway, we've then gotten back together, im hoping to be able to help him turn his life arnd but its hard cos ive never dealt with something like this.. he's poured his heart out like, saying hes useless. hes always telling me hes depressed whenever im not around. he's always saying how he wished he could turn back time, and that his mistakes are something he has to live with forever. he's taken drugs and almost OD. and sometimes when he says this, i don't know what to say to him. im lost :(