Just continue to be supportive and let her know that you understand that coming out of it ( depression ) is a process and not just something she can decide to do and have it happen.

If she doesn't see a therapist, gently suggest that she might benefit from sharing her feelings with someone who specializes in these matters.

Try not to be too critical of her during these times. Even though it may be quite apparent to you that "things really aren't that bad", pointing that out to her may only make her feel that her feelings don't matter and that you couldn't possibly understand. Instead, just keep doing the little things to let her know that you're thinking about her and that show her that you care. Continue to do so even if she doesn't directly acknowledge all of your efforts.

It can be frustrating when your efforts to fulfill your partner fall flat.
Keep dispensing the loving grace and reassure her that you'll see her through this hard time. Tell her that you are certain she will not always feel as she does today. Be a positive motivating force in her life. And give yourself some props for caring about her enough to seek out answers to her troubles in the hope of improving your relationship.

Peace to you both!

-Breezer