I'm positive that it is different for everybody but I feel like I'm honing in on what causes procrastination in myself. After some recent periods of intentional self-discovery, I'm forming the conclusion that perfectionism is the death of my productivity. The common theme with things I procrastinate with is anything where the end is uncertain or I don't have full confidence in my direction. Maybe that's the "fear of failure" you allude to as there is certainly some overlap but I think that's just one notch on the axis for me.
Even though, at an intellectual level, I know that taking any step is better than no step, I freeze up over-analysing whether it's the right step, and fatigue myself. The natural reaction is to procrastinate by taking my attention away from the stress causing decision, even though the stress is totally sourced internally.
But, like I said, it's probably different for a lot of people.
This is me all over. Intellectually I know that doing anything, genuinely any of the possible first steps, is better than nothing but when there are too many first steps I freeze up. I even know from experience that the biggest, sometimes only, hurdle is just starting, but even that knowledge isn't always enough. There is somehow a difference between intellectually knowing that, and really internalising it.
You may need to externalize the responsibility, to have someone else "make" the decision.
For myself I've found someone who is above me in our org, even though they aren't particularly technical. I can take them these problems, they'll hear me explain why I think X is a good direction, or why it's better than Y (even though I'm uncertain of the outcome). They will ask sensible questions (costs, risks etc) and approve the direction, eg "then I want you to pursue option X, and come back to me when you hit unexpected circumstances".
This person can be anyone that you feel some ultimate responsibility towards - in a one-person startup perhaps it's your significant other, as you owe it to them to be efficient and effective (and not wasting time procrastinating).
Even though, at an intellectual level, I know that taking any step is better than no step, I freeze up over-analysing whether it's the right step, and fatigue myself. The natural reaction is to procrastinate by taking my attention away from the stress causing decision, even though the stress is totally sourced internally.
But, like I said, it's probably different for a lot of people.