The amateur needs to think less, the expert needs to think more.
Someone learning a new skill suffers when overloaded. Their journey to competence is impaired by cognition. They need to do first then think about it later, to wait patiently for the pattern and the precepts of the art or craft they’re practising to emerge organically.
This is true in many domains. But what about the art of writing? See, writing is a tricky case because writing requires a target: one has to write about something. So what happens when an expert in one domain attempts to explore that domain using a craft in which they are an amateur? Typically, the expert defaults to expert methods, meaning they think too much.
This is, I believe, why so many find it hard to write about something that, because of decades of accumulated experience and insight, comes easy to them. It is because they are focusing on the wrong thing. For example, an expert asked about his craft, in-person and verbally, can talk for days. But ask that expert to write about his craft without the external aid of conversation or a specific, clearly demarcated prompt, and he’ll likely freeze.
So, if you are someone trying to write about something that comes easily to you in practice, refactor your perception. Focus on that which you know. Or, more specifically, don’t give yourself the time and space to think about writing. Use deadlines, commitments and other stratagems that manufacture urgency. That way, your expertise is given a chance to shine.