"I must think French and write English, be very still and talk wild, act the sage and remain a fool or a dunce." (Henry Miller)
There is a big difference between having an idea and writing it down. This difference is amplified in the case of having an idea in, say, Danish and writing it down in English. The situation becomes even more complicated when the idea is based on reading in English (or, worse, French or German.) Finally, in the act of expression, an idea is often clarified, and once an idea becomes clear to us it begins to change.
Last week, I warned against forgetting the distance between "the surface of your style and the depth of your ideas". (This morning, I must admit, I'm struggling to express what I meant by that.) Two different sentences can express the same idea. One sentence can certainly express an idea more adequately than another. And the same can be said of a group of sentences; one sentence may be a more or less adequate expression of an idea than another depending on the sentences that come before it and after it.
There is no formula for dealing with the disparity between thinking and writing. The only proven method is to think often and to write often, but not to do either for too many hours at a time. Plan your work so that you are thinking, looking, reading, walking, talking, listening and, of course, eating and sleeping, a little every day. This will give your ideas and your prose time to develop at their own pace. It takes time. Let it do so.