The Match Factory Girl. Finnish film, directed by Aki Kaurismaki. I’m pretty sure the title is a shoutout to The Little Match Girl.
Umberto D an Italian neo-realist film by Vittorio De Sica about an old man struggling with poverty.
Locke (2013) with Tom Hardy.
Does Inside Out count? Sure, there’s a cast of a half-dozen major characters, but they’re all fundamentally the same character. Who is going through some serious isolation and depression.
The Machinist.
Would Moon qualify?
Oh, yes, definitely. And the Kevin Spacey-voiced robot with the sad-face display really got to me, I have to admit.
As @ZonexandScout said, that was Locke, starring Tom Hardy, and for me the real drama was about the concrete pour he was responsible for. Not too many movies feature a construction supervisor as the lead character.
Wendy and Lucy
Sandra Bullock in Gravity
One of my very favorite movies, by my favorite director - Vittorio De Sica
(how did I forget THAT?)
Fight Club. The whole film is basically the narrator dealing with isolation and despair.
I don’t know if it’s a “great” film, but I think Beau Is Afraid counts.
How about pretty much any Mad Max film?
Pirosmani
Memento, perhaps?
Taxi Driver has to be the poster child of this genre.
“Falling Down” - kind of the 90s version