When you speak in everyday life, without a script (you rarely have a script), you are improvising. You have the capability to formulate clear and grammatically well formed utterances. Things can go wrong, of course. You might not say exactly what you mean, or you might be less that clear, or make a mistake in what you are saying. You could change your mind in the middle of a sentence and then produce a grammatically confused idea. You can backtrack, or fill in pauses with filler words. But you know how to speak.
A similar thing happens with email. For a routine message, you can simply sit down and write it, and it will be more or less ok.
When writing becomes a struggle, it means that one's relation to it is somehow muddied. I mean, here, for someone who is at the appropriate stage of education for the writing task at hand. Something is interfering with the skill that one possesses already. We call it a "block" for a reason.
Or maybe someone doesn't yet know how to write. It is not a blockage, per se, but a deficiency in skill. Then, it should be possible for the person to learn how to write, assuming competence in grammar (in the Chomsky sense of the term).
What I am trying to say is that you can write the way you want to. You can achieve elegance and concision, if that that is what you are after, and there is no real limit to how far you can get with those goals. When I ask student what their writing goals are, they usually stop at clarity. That is a wonderful starting point as well, because it means that they want to be able to say what they mean.