It's been a while since I've seen it but Cast Away is about being stranded all alone and dealing with loneliness. I have a feeling this isn't what you had in mind.
The Perks of Being a Wallflower is one of my favorite movies. Although he's not without friends the entire movie, I think you might still appreciate it.
The new Joker movie is definitely worth the mention. Although depressing, I really appreciated the movie. It's not actually a superhero movie in case that's deterring you from watching it.
Although I haven't seen it yet, Lost In Translation is near the top of my watchlist. It might be up your alley.