A relationship with depression?

My ex partner suffers from depression, and it has really been a roller coaster ride these last 3 years, at first all seemed well and she seemed happy and would try in our relationship. Then the problems started, one minute she would be up and want babies, marriage even and a future with me. Then the next she would be down, and want nothing to do with me, be on her own and tell me she doesn't love me. The final straw came about a week or so ago. We had a really great weekend, she told me she had always loved me and I knew that deep down she always had, we were like teenagers walking in the park hand in hand. Then the next day she seemed really fed up, and on the Tuesday ended us with no real reason at all. Just said she was miserable, wanted to be on her own with her young daughter, that she wasn't really happy at the weekend at all, and that she didn't love me or want to be with me. I suspect that her depression is a lot more serious than the doctors have diagnosed, I live in hope that we can patch things up even though I have moved out. I love her so much and it's hard to let go because I know that when she is happy things are good and its worth a million to see her like that, but when she is down it is like walking on egg shells, or riding a big wave on the ocean. Any suggestions at all. I can't give her up, even though she has made me feel so low at times.