Short answer: Delete the task.

Longer answer:
Regardless of how important or decisive the task you keep postponing is, sometimes the mere existence of that task prevents you from doing anything else. If, after several days, you still can’t handle the main stressful task, delete it. Don’t do it, and make sure you absolutely won’t do it (cancel the appointment, project, task, or whatever it is). By removing that task, your mental peace will be restored, and you may gain the immediate capacity to tackle it in the future.