In short, it aggravates my perfectionism to such a degree that the project spirals and scope-creeps until it becomes impossible.
It's like every detail affects the entire project — a multidimensional knot that I have to map and untangle before ever starting. I see how each design choice has knock-on effects down the line and I get stuck in decision paralysis.
I need to get back to focusing only on the next step, and accepting that multiple iterations will likely be needed to get it all right.