I know hardly anything about formal meditation, or any formal theories on managing depression for that matter, but I would submit that there is truth to javajoshs' larger point, at least as I read it.
A big part of managing my mood is to do something like what he says. I don't know if you can, with enough work, literally choose your thoughts---I certainly still can't. But you do have some amount of control over your attention, and you can train yourself to have more. You can learn to recognize your negative thought patterns, to pick out when your thoughts are trash, and to let those thoughts float away, rather than latching on to them. You can learn not to focus on them and not to wallow in them for hours, days, or weeks. Once you are more adept at letting these thoughts come in and out of your conscious attention, you will find that there is again space for happier, more constructive thoughts---latch on to these and they will multiply.
I still can't avoid the occasional storm, nor can I be sure that the full-fledged, incapacitating, depression won't come back sometime, but I feel that I have made great progress. I'm much happier than I was a year ago, or two years ago. I think such a strategy could help a lot of people, and maybe you too. It is, however, hardest to learn how to do this when you are in the midst of a depression. Progress is slow, and requires the sort of effort that is painfully difficult to muster, when you often can't even summon the concentration to bathe or eat. You have to go one step at a time. You can't put too many expectations on yourself, as painful as it is to accept that you will lose even more time to your depression. You can't rush it. But, after a bit, you will (I think---take all this with a IMO at the top) start to feel like you are building up a resistance to your depressive thoughts, and the momentum will pick up. One day you have a feeling, say of really-moving-empathy, for the first time since you can remember. After a little more time, you look back and realize you were more or less happy all week. Then maybe a week turns into 6 months.
I understand your skepticism, but I urge you not to be so dismissive. Other people have been through what you are going through, even though it doesn't feel like it is possible. Those who have recovered probably don't fully understand what led to their recovery (I don't), but they do bring back some insights. Everyone on this thread is just trying to express those insights. We can't say exactly what would work for you, or even fully express what we experienced, but if you dig deeply enough, I think you will find a lot of truth in much of the advice offered. I think there is a lot of truth in what javajosh said. I guess your response just struck a chord with me, because it sounds so much like me. I remember being dismissive of everything, because I felt like I had tried everything, and I felt like no one really understood. When people said anything like "You just need to choose to be happy", I decided that they were just callous or obtuse. While some of them probably were, I'm pretty sure some of them understood. It’s just that there is this barrier--- in the same way it is hard, while you are depressed, to remember being happy, it can be very hard, while you are happy, to remember how painful it was to be depressed. Perhaps this barrier can be a goal?
Anyway, something in you does have to "decide" to get better, to learn to manage the depression. I'm not sure "choice" or "decide" is the right way to say it, but sometimes language is a blunt instrument. It's more like slowly learning to commit yourself, in each moment, to attending to your thoughts. It's like learning to guide or steer your mind, as well as you can with the handles you can find, to a better place, all the while searching for more and more handles. I hesitate to use words with negative connotations, but language is a blunt instrument, so I'll say sometimes it even feels like manipulating yourself, or conspiring against certain parts of yourself. In this process, you have to accept that there will be many failures, and you have to practice tenacity---learn to keep getting up (and then getting up gets easier!).
In the end, I don't know your personal struggles. Perhaps they really are far more intense than mine. And I tried really hard not to sound dismissive of you, myself. But, I really want to urge you, and anyone else who is really down, to try to glean from what I, and others, have written. Try to tease out what it is that we are trying to say. Please just give it a shot for a week or two, instead of reflexively lashing out. I'm not trolling you, and I don't think javajosh was either.
I wish you the best of luck with all I've got.