Loz,
I'm sorry to hear what you're going through. Depression is a horrible fact of life and one that all of us will have to deal with at some point in our lives, either for ourselves or for someone we care about.
I can't really offer much help, except the fact that you're not alone, and many people in the world have felt, or are feeling the same way as you are right now. And many recover.
Personally, I have recently recovered from a 2 and a half year bout of depression, beginning just before my final school year, and finishing around May this year. I was never really diagnosed but showed many symptoms of a condition that has since been much publicized around the internet and in the media. Depression is a horrible disease because for no known reason it eats away at your quality of life, and people don't recognise it as a disease. Instead, depressed people get told to cheer up because they're problems are silly. They're told that there are people much worse off than them, but this doesn't help someone who is depressed. Depressed people can also end up being ostrasized, and this obviously doesn't help the cause.
Nowadays, I feel like I'm a much better person for coming out the other side. I used to feel like I was weak, I thought I was unbelievably weak because I couldn't kill myself, or sort things out or whatever, but now I realise that far from being weak, I was strong, because I battled it out, and kept battling.
The number one factor in getting through it was my support group - my family, and a couple of close friends. Loz, I suggest that if you have this support available, you turn to these people if you haven't already, if they matter, they'll be there for you. They might not be able to actively help you as in solve all of your problems, but they'll be there to catch you when you fall. If you don't have people like this available, then I know there are plenty of structured support groups out there. Go along, talk to people, you never know, it might help, and if not, well it isn't going to hurt.
I don't know much about Bipolar/manic depression, other than the fact that it's mood swings from extreme depression, to mania. I do know that it's treatable with medication and other remedies, but I trust you've checked these things out already.
Finally, I'd like to say a few things that may seem like cliches, and probably won't help, because nothing I can say is new, but which I've found to be true. We're all here for a reason, and I DON'T mean that in a religious way. I think that every single action that we take shapes the world forever we do all have something to offer to the world, please don't consider taking away from us whatever it is that you have to offer. Secondly, everything happens for a reason. Again, I don't mean this in a religious way. If times are bad, living through them can only make you a better and stronger person.
I'm sorry that I can't say anything to really help, I think you're a very strong person for bringing this up on a messageboard like this one, and that can only be a good thing. I wish you all the best in your recovery.