Is the mantra for doing quality work at high quantity...
It is a funny thing that the more you worry about something, the more you think on what is to come, what you aren't doing that very moment - the more difficult it becomes, the more frustrating, the less clear.
You get stuck in the minutiae of implementation.
The truth is our minds cannot accurately simulate a reliable scenario to realistic levels. So when you play those things out in your head, when you start to worry about negative factors that come into the equation - you overload your brain, so much that it can't think straight.
We are doing machines. We think, then commit to an action. That's about as much as we should be overloading our heads with. The act of creation allows you to think clearly.
When this stream is broken, by overloading it with assumptions and non-rational thought, you stop it from doing what it does best - the actual doing of the thing...
Know your limits, then plan within them. Know from a distance what you are capable of when you get there. Then do not think about the planning of it when you are dong it, and vice-versa, don't think of the implementation when you aren't there yet.
Worry about it when you get there.