You are convinced and also convince yourself that the things you want to do or need to do are too enormous in scope to deal with at all.
Example 1: ALL AT ONCE
"I have to accomplish the whole project at one time, in one sitting."
Break each project down into small, discrete, and manageable units which you can complete one step at at time, and pepper in pauses for the sole purpose of reflecting on and appreciating each step.
Example 2: FRAGMENTATION
"I can't start or continue my new or current project because there are other things I haven't gotten around to doing or finishing yet."
Pick one thing and work on it. Focus on what you are up to by doing it.