I agree very much with MediumMoodSwing. I hope there was a formula that would work with all of us so nobody had to endure any pain!! But unfortunately there isn't and the best we can do is keep trying until we find something that helps us, even if is just a little bit, to make life a tad easier.
Like you I have tried so many things, low carbing, vegetarian diet, exercising, supplementing, mindfulness meditation, prayer, drawing, writing, and nothing has 'cured' me, but I would be lying if I said all these things together had not made a difference. It may be slight improvements here and there, but overall when I look how much I accomplish and get done these days compared to let's say a year ago, is quite clear that overall I am doing much better and that I am functioning much better. Sounds to me like you are already doing all the 'right' things, but perhaps might be worth to keep trying with the vitamin B, maybe some other brands? I know how frustrating it can be when you go from one doctor to the other and they tell you that all your levels are 'fine' and that you are ok (so it must be all in my head, right?!). I still haven't found the right doctor but at least I've discovered I do have some nutritional deficiencies and I am tackling those and I hope that will make a difference as well. But yes, is very frustrating with doctors nowdays.
I am sorry to read you have not found any relief yet here and I really hope you'll find what works for you soon. I'm obviously not there myself, and I may never be, perhaps I will just learn how to 'manage' my depression (like someone mentioned on another thread), learn and experiment with new tools such as CBT so I can cope better on the bad days. But that's better than being depressed and hopeless.
Good luck and do not give up!!