A plan so detailed it takes a month is sometimes just a very organized way of not doing it.

Planning is comfortable because it feels like the work while carrying none of the risk. Past a certain point the extra detail is not reducing uncertainty, it is deferring the moment when reality gets a say. The tell is when the plan keeps expanding and the start date keeps moving.

Give planning a fixed budget, say two hours or two days, and start when it runs out regardless of how ready you feel. Plan the first step properly and leave the rest rough. Most of what you would have planned is going to change once you begin anyway.