It's okay, nobody's head works properly when they're depressed! (I forget how to spell, and I'm a writer fgs!!) Cadbury was bang on with what she said about hurting your hand - all your head space is used up with the injury. Same with depression.
I thinks it's a vv good idea to talk to HR. Returning to work's going to be sticky anyway, while you're poorly, so go and ask them [a] what the mat pay surplus would be, and [b] how long you can have to pay it back, as you're going to be unwell. I don't want to create false hopes, but there is a small chance they'll write it off.
One important piece of advice for people going into a severe depression is: don't play 'coping'. I did it for years 
and it really held me back. There's nothing bad, shameful or even unusual about what you're going through. You have an illness. The feelings of all-round crapness are the symptoms. Let them show when you're asking for support, just as you wouldn't mind sneezing when you're telling people you've got a stinking cold!
Do go outside, even for a little bit. Find something nice and pay attention to it. Just a fresh flower, or a cloud or a giggling child perhaps. It's good for your brain to be reminded there's some pleasant stuff around. Oh, and chocolate! It's antidepressant!