It's a bit like getting further and further out of shape that way, because of the changes of thought patterns and self-image and socialization patterns etc - the further you go the more the very things you need to get out are being undercut.
(And yet, just like that example, you can always start where you are at, and sometimes even a bit of change can be more dramatic and exhilarating than you think it would be - perhaps walking up the stairs without getting winded feels like a huge victory, if you aren't focused on how long it will be until you run that marathon)
In my own experience, and in working with people with severe depression, I have found two things to be almost universal. One is that the pathway out involves better noticing of small shifts in consciousness in the present moment - depression is a flatland, yet even within it there are small tiny windows of light that can be noticed and built on.
The other is that, and this especially applies to people that live inside their head, are very smart, very analytical, etc...the body itself provides a pathway out. Without even getting into somatic psychology, the act of doing things where one is experiencing felt sensation in the body provides a ground for consciousness outside of the grey hazy thought world.