There is definitely such a thing as being over-prepared.
When preparing to write something, it is easy to get caught in an endless cycle of pre-writing: character bios, relationship charts, detailed step outlines, and so on.
Sometimes you just need to type “Fade In:”
I don’t advocate flying completely blind. A certain degree of preparation will help you gather momentum, as well as give you something to fall back on and review if you get lost on your way towards the end of a draft. This doesn’t mean there are certain necessary steps that must be completed.
You only need to take a deep breath and decide to get busy writing.
What it takes to get to this point is different for everyone, and takes some trial and error to discover. This will mean false starts. This will mean having to go back and begin again. That’s fine. It’s all part of the process.