There is a very specific kind of frustration that student writers know well: you write a sentence, immediately decide it is bad, delete it, rewrite it, decide the new version is also bad, and delete that too. An hour passes and you have half a paragraph. The problem is not talent. The problem is doing two different jobs at once.
Drafting and editing use different mental modes
Drafting is generative - you are producing raw material without judgment. Editing is critical - you are assessing and refining what already exists. When you switch between these mid-sentence, neither one works properly. The editor in your head interrupts the writer before a single idea gets fully formed. What comes out is cautious and thin.
Separating the two stages with a physical rule
One effective approach: write with the screen dimmed or the font turned white so you literally cannot see what you just typed. You can also just commit to a rule - no backspace key for the first 20 minutes of a session. These are not gimmicks. They remove the option to edit prematurely so your draft brain can actually run.
A writing tutor named Oswin started teaching this split to his group of second-year students. Within four weeks, the average first-draft length in their assignments increased significantly. The drafts were messier, yes - but they had actual ideas in them, which gave the students real material to work with during revision.
A messy complete draft is always more useful than a polished half-paragraph. Let the draft be bad. Fix it later.