Overthinking creates thousands of hypothetical situations in your imagination. You don’t have to solve them all. (You can’t list all the possibilities even if you want to).
I agree you should have a well-thought-out plan. But you have to stop somewhere and start acting on it at some point.
Take a decision. Move a step forward.
It will end the branches of old possibilities and give rise to new ones.
You’ll have new problems to think about and solve — once you move forward.
You’ll be stuck with your old never-happening problems if you keep on overthinking about them.
1 step.
That’s all you have to do today. 1 meaningful step. Ahead.