I experience the same phenomenon in spite of not experiencing anxiety. (That's not 100% true, I did experience completely disassociated anxiety once.)

The most interesting case is that I spent about five months writing a new tabletop game after the beta of the current version of D&D made me annoyed. (It was when they started phasing feats out, eliminating yet another chunk of character customization.)

Five months and a novel's worth of writing in, I started planning ahead. As soon as I set goals for myself, I stopped enjoying working on it. I pushed through writing 250 spells over two months, and progress has been sporadic since then.

I don't think anxiety is the issue. I think it's something related to goal-oriented behaviors; the short view and long view fighting each other.

ETA: Thinking about it, I experience exactly the same thing WRT my daily work. If I receive an e-mail with something to do, I'll immediately hop on it, and wrap the task up. If I have a long-term project, I'll procrastinate. A task that enters my immediate list of things to do carries little or no internal resistance; the same task, attached to any kind of prior planning ahead on my part, requires substantial effort to undertake.