Was probably adapted to become the 2018 Legion netfix series because the plot is almost identical.
It's about a man in a psychiatric ward possessed by an alien force...
The Bridge by Iain Banks (published 1986) ? Has three characters that are inter-related sort-of in the way. One of them is 'the Barbarian'.
Could be 1903 novel by author John Antoine Nau, Enemy Force (Force Ennemie) republished in 2010. It won the French Goncourt prize in 1903.
Was probably adapted to become the 2018 Legion netfix series because the plot is almost identical.
It's about a man in a psychiatric ward possessed by an alien force...
Force ennemie - Wikipedia
en.m.wikipedia.org
How about a long shot....
The main character is a young woman in a psychiatric hospital who retreats into a different world and language.
"I Never Promised You a Rose Garden" by Joanne Greenberg?
I Never Promised You a Rose Garden (novel) - Wikipedia
en.wikipedia.org
and here's some comments:
I Never Promised You a Rose Garden (novel)
I Never Promised You a Rose Garden (1964) is a semi-autobiographical novel by Joanne Greenberg, written under the pen name of Hannah Green. It was made into a film in 1977 and a play in 2004. [hide] *1 Plot summary 2 Inspiration 3 See also 4 References 5 External links I Never Promised You a...ultimatepopculture.fandom.com
I thought this was an unusually insightful book, based on real-life experience.
The psychiatrist in it, Freda Fromm-Reichmann, was by all accounts in real life a remarkable individual .