What to do with my depressed girlfriend?

Hi folks, me and my girlfriend have been together now coming up 3 years, during these three years she has had bouts of depression here and there. Due to the fact she lost a child around 9 years ago shortly after he was born and the way her ex boyfriend treated her for 6 years. After the birth of our son last year she was diagnosed with post natal depression. The months went by and gradually got better and we moved into a new house in February this year and she couldnt of been happier. But in the last week or so she said the depression has kicked in (she does take medication) again, I do not know how to handle this situation whatsoever, her depression brings me down with her when normally I am a cheery/happy guy. There are days when she hardly speaks to me, gets upset at the silliest of things and to be honest it feels although the depression is a third member of our relationship. It is always a guessing game wondering what mood she will be in, dont ge me wrong its great when shes not depressed but I always know it will come back and affect her. If this realtionship makes me miserable sometimes is it worth it? i really truly love her, but i dont think this is something i can fight (dont know if thats the right word to use) for the rest of my life.