At some point some people come to a breaking point... what used to be known as a "nervous breakdown". I think of it as a progressive paralysis, a psychological (not physical) inability to continue.
The lucky ones can "just snap out of it", take a break and come back refreshed, or switch jobs and everything's ok again.. or at least bearable.
For the rest of us, it's not so easy. Yet, paradoxically, and this can be a source of tremendous guilt, "getting over it" should be the easiest thing in the world.
"You just..." fill in the blank.. pick up the phone, dial the number, take a shower, brush your teeth, get out of bed, put on your clothes, go to work, do what you know how to do and what you've been doing for X number of years all along, deal with your colleagues, deal with your boss, eat your lunch, eat your dinner, go back to sleep, wake up for another day, rinse and repeat day in and day out again and again...
Some can manage it, but for some it gets to be too much.. simple things, such simple things... most can do it, their whole lives, they just do it, why can't we?