I believe that JRR Tolkien suffered what could be termed a 'nervous breakdown' in 1945-6; after taking-up the Merton Professorship of English Language and Literature in June of 1945, and at exactly the time when he was writing the Notion Club Papers (NCPs). The Notion Club Papers is therefore itself an indirect source of evidence about Tolkien's state of mind.
This period of 1945-6 was also associated with an apparent marital crisis, during which Tolkien (with his son Christopher) and his wife separated for some weeks. The evidence suggests Tolkien's psychological problems were building-up to become severe by December 1945, peaking in March and
April of 1946, and resolving in July of 1946.
My impression is that this breakdown was mostly a matter of anxiety and depression brought-on by overwork and stress.
My guess is that the nervous breakdown experience of late 1945-1946 had a permanent effect on Tolkien - and that the effect was beneficial to his writing. On the one hand he was able to write with increased emotional depth. More speculatively; it is possible that the experience of his 'self-therapy' in writing the Notion Club Papers was able to give him surer access to altered states of consciousness, especially dreams, and these provided a source of other-worldly sub-creative reality to the Lord of the Rings.
Without the nervous breakdown of 1945-6, and without the experience of writing the Notion Club Papers - The Lord of the Rings would have been a different, and probably lesser, book.
Read the evidence at The Notion Club Papers blog...