I understand this pretty clearly, and so I think I can draw a useful distinction.

A little bit of anxiety or worrying is a useful signal to my whole mind to sharpen my focus. For brief stresses - with projected duration of a week or less, usually - this is good, and I can actually experience it as a pleasant challenge to rise to.

If that same "little bit of anxiety" stretches out over weeks or months, though, I become numb to the motivation, but not to the negative effects of worry. Also, if I'm worried enough about something that might take a while to finish - more than about three or four days - then I'm prone to shutting down entirely. It's as if some part of me is convinced that it is important to worry for its own sake, and I'll wind up spending up to three or four hours a day doing nothing but staring off into space, worrying unproductively. This sort of worry is unhelpful, and demotivating, and what I'm trying to curb.