Many of us in the US do not work with one foreign language alone, but teach, read, and write in three- our native American English and two or more others. These may be related (e.g., French and Spanish) or not (French and German). The problems there become even more complicated, with interference at multiple levels- syntax and emotion as well as lexicon. "Turning off" is not totally possible. I would suggest looking into these as well.
Your statement "my writing style... simply had to become more French" is deceptively simple, and it hides a myriad of
issues, big and small, which come into play in writing. I am
a native English speaker, bi-lingual Italian, and teach English
as a second language. Over time I've discovered that I need to
double check my writing in both languages. The English can have an
overlay of Italian and the Italian a shade of English. This can be (not a very scientific word) fun for me; but it certainly takes a lot of time. Congratulations on completion of your book! That was quite an effort.
François Grosjean, Ph.D., is Emeritus Professor of psycholinguistics at the University of Neuchâtel, Switzerland and the author of Bilingual: Life and Reality, among other books.
Aneta Pavlenko, Ph.D., is Professor of Applied Linguistics at Temple University, Philadelphia, and author of The Bilingual Mind (2014) and many other books.