You simultaneously feel like you're too burned out to be able to do some entirely new thing, if you can even muster the energy to engage in the switching process (e.g. job hunting, etc.), and the familiarity of the thing that's burning you out brings its own kind of perverse comfort.
However, now that I've been deeeeeep in it for a couple years (still doing the thing that's burning me the whole time and even now) I've developed an appreciation for what it probably really is... existential malaise. That's a really useful thing to experience. Because it can be used to redefine a superficial value system you've adopted, to get to this point of dread in the first place, toward things you do actually find personally fulfilling for their own sake. Sometimes in dramatic fashion.