Welcome to the forum.
Geting to the bottom of it -> learn about depression.
See the black dog tribe, blogs, here on sane.
And the resources tab
mind also have a good website
You also have your own experience, everyone's mood changes, so tune in to that. Notice how sometimes it won't budge, but at others positive thinking can help.
Fix it -> just be patient as you learn more you will be able to know what to do. Try not to be on eggshells, be confident, it's OK that she feels depressed when you feel fine, you are for support but she is responsible for how she behaves, not you.
Hugs and iLoveYous are free to give.