From my understanding manic depression is actually Bi-polar. Just a different way of saying it. My ex girlfriend has been on medication for long time , think she said it usually accours early to mid twenties but can start before this.
Type 1 and 2 there are subtle differences. type 1 disorder from memory is most common, suffering the high,s and lows - manic depression and mania, type 2 i think means a more hypermanic episodes with major depression . In between all this you can suffer both together hence the BI-polar. An inbalance of mood control within the brain.

Apparently her GP said there,s probably lots of people unrecognised . Some of her symtoms would be to buy lots of the same things for example. I,d say Nicky boarders a manic state more than depressive.

As for being depressed , mentioned above, very difficult to acheive but keeping busy will definately help the mind , and talking to people can make a big difference.
I used to use a theory that no matter how bad i felt, someone has really got a lot worse situ than yourself. Also your not alone, i think feeling alone is a subtle and devastating inner feeling