I think that so much advice about the small things like forcing a quick change of scene with a walk / talk / book etc, making a small change to routine and all these things is so much more effective than so many people think it is - but the trouble is many people can't accept their very unique and special brand of helplessness, hopelessness and pain is actually not that unique at all. It's part of the human condition to think that even in our misery we are somehow so apart and unique from others that 'normal' advice couldn't possibly work on us. That's the thing about pain and loneliness, it makes you feel like you are the only one.
Thing is though all the small things do work when put together. Life is only a series of individual moments, strung together by the common threads you choose to weave them with. That's pretty much it. It's far more mundane and far more spectacular than most people realise.
The day I started to get better from these sorts of feelings of hopelessness was when I realised I wasn't so special that my problems were so insurmountable, that I wasn't so unique as to be the exception to the rule of what helps basically anyone who gives it a try improve their life (exercise, better sleep, better diet, new activities, keeping the brain active, learning how to challenge negative thoughts etc).
And when I also accepted that given that I was not on my deathbed, or locked in prison or something I was in a fucking fortunate position to be able to effect small changes in my life and to stop being such a fucking child about it, stamping my foot and saying life isn't fair and my life in particular was the one unique life that could never be helped.