There is only one language-processing centre in anyone’s brain. We can either listen, speak, write, or read at the same time. Problem is that of you start correcting yourself, you are unconsciously reading what you are thinking and writing, interrupting the free flow of thoughts.
That great idea you just had is gone in a heartbeat as a result of focusing on the perfect word or sentence.
Thoughts are often unstructured. Write them down as they come. Do not worry about orthographic correctness or choice of words.
Turn off any distractors. This includes real-time spelling-and grammar-checker. Often, they’ll stop you dead in your tracks by competing for your attention.
You can always come back to revise, organise, and flesh out your train of thought later.
That way you not only will create intellectual property much faster, but your audience will value your authenticity.