Hi nytbd, your situation somewhat mirrors my own. I'd been living with my partner for five happy years, when in april she told me all of a sudden that she wasn't in love with me anymore; and that she felt empty and numb inside. It happened quite suddenly, and immediately after announcing this she indicated that she wanted to move out and start over.
The sentiments she expressed at that point, i'm sure, are not dissimilar to those of your partners. She said that there was nothing wrong with our relationship, she just didn't feel 'in love' anymore. She said that even though she didn't think there was any point going on with this empty feeling inside, that she was worried she wouldn't be happy with anyone else.
After asking her for some time to reconsider, we tried sticking together for a month after that to see if things changed (at that stage I didn't suspect depression) - they didn't, and I lapsed into a state of demoralisation that closely resembled depression... Couldn't eat, couldn't sleep, couldn't concentrate.. I was absolutely miserable. This of course really made things quite bad for the two of us.
I just plain couldn't understand why she couldn't explain how she felt. Worse still, if I were to push the questions, she would stress out, or be locked down by anxiety, or something - to the point where she'd just stonewall and stop talking.
She's been moved out for about a month now - however we've since discovered (over the past two months) that she has good days where she does have hope for the relationship, and bad days, where she feels that the future's quite bleak. She thinks of our relationship with a mixture of hope and dread. She can't seem to associate any good emotions with memories of us - five years' worth! So reminiscing doesn't quite help either.
The most important lesson i've learned, is not to pressure her. This has certainly helped in our relationship; believe me, when I took the same course you described, it drove her away too.
I learned to set boundaries - not to not love her, but to love myself first - and be the person that she closely resembles who she fell in love with a time ago. When I started feeling better and more confident around her, she started feeling more comfortable around me.
I still haven't heard an 'i love you' from her since april - but talk of moving out and starting over has turned to talk of 'hang in there'. It's not easy, but I try not to address her depression and neither do I try to extract some sort of affirmation of love from her because I understand that she is currently incapable of feeling these emotions healthily enough, and to ask would only mean an honest reply from her of 'i don't know if I love you', and demoralisation. This much I can control.
All I can tell her at the moment is that I love her unconditionally. Not telling her 'i love you' every time we meet as that would pressure her (we're walking on eggshells, remember?

). But as long as you have the strength and love to be there, let her know that you understand. Let her know that you're not angry with her, and that it - the depression - is happening to you both. And the both of you have to fight it.
Your partner has agreed to see a therapist, and that's a great first step. Acknowledging the issue is tremendous - my partner is seeing a therapist too, but we don't have too much faith in this particular therapist at the moment. As such, despite all i've read and learned about depression and its symptoms, as well as from the people i've spoken to - i'm pretty sure my partner's got dysthymia - but not having a health professional tell her that means she can still be in denial, and assume that the decisions she makes are hers, and the feelings she has are hers - and not caused by depression.
Hang in there - take time out to heal, tell her you love her and that you'll be there for her. But remove yourself from demoralising situations first - talk about other things. Be less selfish - ugly word, but i've been in this situation too - don't push at the depression issue with her in hopes of hearing what you want to hear from her. Believe me, the number of times i've hopefully initiated a conversation about her feelings, depression etc in hopes of "you're right, I love you, and i'm just depressed. It will be ok, i'm already feeling better." - too many to count, all have just ruined what might have been a nice evening because all I get back are "i don't know" and "i don't want to talk about it", or just stone cold silence.
Instead, help her seek help

it's really hard, I know. I'm in exactly the same situation, and all I can do is hope for the best.
I'm hoping to get my partner to speak with a friend of mine who's suffered depression, and hopefully she'll be able to gain an understanding that it's not happening only to her - and that she's not at fault, and that this bleakness can be lifted. I'm also going to suggest she see a gp recommended by people who've been through depression (this is important!) - maybe with a little medication (hate the stigma that comes with that word; why not 'dietary supplements'?) she'll be able to lift her head above the clouds, think a little clearer, and hopefully - feel a little clearer too.
My heart goes out to you, man. Hang tough!