Solitude gives your thoughts enough space to stop performing.

Thoughts behave differently with an audience, even an imagined one. They arrange themselves to be defensible, to sound reasonable, to be the sort of thing you would want to have thought. Alone and unhurried, they stop performing and start being accurate, which is often less flattering and much more useful.

Try thinking on paper with the certainty that nobody will read it. Write badly on purpose if that helps. What comes out under those conditions is usually closer to what you actually think than anything you would say aloud, and it is the material worth working with.