The task is too vague to start

"Work on the dissertation" is not a task, it is a topic. Nobody can begin it, because there is no identifiable first physical action. The mind reads an unstartable item as a threat and quietly routes around it.

Rewrite anything you have avoided twice until the first step is something you could do in ten minutes without deciding anything else: open last week's draft and reread the middle section. Find three papers and save them to one folder. The rewrite is usually the whole intervention.