Do you ever start a task and then realize you have been thinking of something far down the line which doesn't actually relate to the current task at hand? Overthinking a project is normal, so get over it. Write down all of your thoughts or concerns for the project, that way you don't have to spend time and energy reminding yourself of what the concerns are.
Cutting through all of this mind clutter is how to get to the core of your project. Always ask yourself, what is the goal? Does "x" help me achieve this goal? Even as a designer, it can be overwhelming when you think of all of the possibilities available for you to create. Clear your mind and keep going back to addressing the goal, it will help navigate in the right direction.