Books where the Protagonist is Alone?

This feels hard to find, but as a kid I really liked books where the main character was by themself and overcame struggles, think Hatchet or Island of the Blue Dolphins.

I’m looking for something a bit similar, but for an older audience. It doesn’t necessarily have to be wilderness survival either. I just want to be inside the character’s head the whole time. Bonus points if the story isn’t super depressing and has a more optimistic feel.

I know these types are hard to pull off in fiction because of the need for a really strong main character, so give me your best ideas please!

by sincerelysunshine