Depressed boyfriend

my boyfriend is depressed and its putting the biggest strain to ever exist on our relationship. We've only been together a year and half of that has been plagued by the depression. It appeared to start when his ex girlfriend of 7 years, (who he broke up with to be with me) left his life for good. She moved out, they don't speak etcetera just clean cut. He became distant, even more lethargic than he was and avoided coming to bed with me by watching TV and playing with his phone downstairs. All of this blew up and we had a massive argument where the reason behind his behaviour (grieving his ex in the house they shared which he was by now sharing with me) came to light. He told me, I was understanding and said I would stay because I loved him and wanted to help him through it rather than abandon him. A few weeks later I looked through his emails and noticed he had joined plenty of fish. This obviously put me in a blind rage as I waited for him to return home. On his arrival I recited s thread of messages to him. he didn't appear sorry as we 'weren't together'. I left the house that week as I didn't think joining plenty of fish had anything to do with depression and thought he was full of crap. We made up and went back to talking and having sleepovers occasionally. I still loved him and I am weak. Until a few weeks later when his off and on 'we're not together but we have to still act like it' attitude grated on me too much, I tried to cut him off and ignore him. I wanted him to stop taking me for granted and it kind of worked because he was constantly messaging, all the while I ignored. Of course we made up again (love him, weak). Two weeks into this period, I discovered that whilst we were 'not talking' he had had sex with one of my co workers who also used to be his coworker, a beautiful leggy woman. She is the opposite of me pretty much. Anyway when I heard this I was absolutely finished with him. Until he talked me around in to being friends again. I can't seem to stay away from him. It's literally the whole Friends 'we were on a break!' Scenario. This was about 5 months ago. From then until now I have finished it twice and given up twice. He's lazy, has mood swings, one minute he adores the dirt in the cracks of my shoes, the next he couldn't care if I moved my butt all the way to Australia and never saw him again. He is incredibly stubborn and has a terrible diet. I know he loves me. Despite all this, I know it. Very recently I've been seriously thinking about dropping him because I am fast becoming the miserable cockroach I was in college. I'm becoming depressed too. I don't know where it's all supposed to get too much for you and you bail to save yourself. I can't give up on him but sometimes I feel like I need to. The other night we had a few drinks and I cried and told him every detail of how all of this makes me feel and he cried with me and revealed that he is back in a place where he doesn't care if he lives or dies. I believe most of the above is due to his depressive nature. Because he's stubborn he won't seek help. He won't TRY to change his diet to see if that helps. He won't TRY to take certain daily vitamins. I keep suggesting small things he can do to TRY and turn it all around and he refuses to. Because he also refuses to go to the doctor about it because the medication won't make anything real and nothing he will feel on these tablets will be REAL. I'm truly suffocating but I can't abandon him. If anyone has any suggestions on how I might get around his stubbornness that would be great. I want it to back off and let him come out and have hope and to know that life has so much to offer. I want him to give me baby's and Feel again like he needs and wants to be alive.