It’s a futuristic sci fi movie so it might not be what you’re looking for. Loneliness and alienation are central themes, though
Moon
Eternal Sunshine of the Spotless Mind. Explore how it feels when you deliberately choose to lose a relationship.
The main character is an unemployed mathematician with schizoid personality disorder whose social interactions are mostly with his old professor and the people in his apartment building. The movie is a thriller, but the main character’s isolation from the rest of the world is rather stark. This is helped by the fact that it was filmed in high-contrast black and white.