Oh lordy, some of the comments on this thread are not helpful. That's the most I'm bothering to say about them.
Basically, I have been in your girlfriends situation, almost to the letter.
I've had major clinical depression on and off for well over fifteen years (although it hasn't reared it's head for the past five) due to some nasty things happening, no need to go into that here, you already know what depression is like, and mine was almost identical to your girlfriends.
I found I got a LOT worse when I was with someone. I knew that I was making their life hell, and that made me feel SO much worse. I used to self harm in order to try to cope without upsetting anyone, but on the few occasions when I was 'caught' it was terrible. I barely felt anything, and to be honest I don't know whether I really did love my ex - I felt so little, all I knew was that I didn't mind being near him.
In the end I realised that we only spent time together because that was what we had done for the past four years. He tried his best to make me feel better, but (loathe as I am to admit it) having someone pity me did stop me trying to drag myself out of my pit. It is important to have someone to lean on, but the trappings and responsibilities of being in a relationship are too much to cope with when you are really depressed. You don't have to leave her totally, but it would be best for both of you to pull back a little, but let her know that you are there when she really needs you, any time of day or night.
Medication-wise, it took fifteen goes to find one that worked for me. There are other treatments available too, sometimes counselling can't do the job alone.
Usually depression is either caused biologically or psychologically. I know mine is at least partially biological, due to the high incidence of it in my family, plus it's re-emergence over a long period.
Have you any idea what caused this depression?
I also agree that distractions tend to work very well, especially as from what you have said this is something that has had a marked effect. Have you thought about tai-chi or similar? Exercise helps to release chemicals that combat depression, and can actually be prescribed by a GP as a treatment. St. Johns Wort might be worth a try too, as long as she isn't on the pill.
Avoid regression therapy. Not a good idea. Thats how False Memory Syndrome began. Plus if the depressive episode is biological or due to a recent trauma there is literally no point to it.
My ex and I are good friends now, and I can't believe what I put him through. I am pretty sure if we were still together I wouldn't have been able to get out of my 'pit', so to speak - it doesn't make sense, having someone love you and be there for you should help, but it honestly made things worse. It sounds as though she knows what you are going through, and that won't help her at all. The main aspects of most relationships aren't present with you two right now, so pulling back a little is something that I really think would help to lessen the pressure on both of you - just make sure that she knows that it doesn't have to be permanent, you won't go off with anyone else (or at least will warn her first, I didn't have that luxury), and that you are still there for her whenever she needs it.
Saying all that, there is the small but present chance that this might make her worse. Keep an eye on her and see how it goes. It might just be hard for her to hear, it might be too much. I honestly think, personally, that it will help her, but everyone is different and I'm certain that you know her best - if there are any signs that pulling back might be dangerous, then use your own judgement.
Feel free to contact me if you have any questions, as I said I've been in her shoes for a long time, so it might help to speak to someone if she is not able to. I'm online at least a couple of times a week so I can give you my mail if you need someone to chat to.