I'm writing a story where the protagonist is the strongest but because of that, it causes them to be lonely and have self-doubt about themselves. What are more ideas like that?

Hercules. Reacher. Dr. Manhattan from Watchmen. Hamlet. The Incredible Hulk. Major Motoko Kusanagi. Connor Macleod. The Doctor from Dr. Who. Lisbeth Salander. John Wick.

This is a trope nearly as old as the Fall from Grace. It’s hard to swing a cat in a library without pissing off the cat, and also hitting a dozen books that make use of that trope.

It’s even part of the standard Hero’s Journey. The classic hero who saves the world is forever set apart, usually because of the trials they faced in their story, from becoming part of that world (see Frodo Baggins, Dirty Harry, the dude whose name I don’t remember from The Good, the Bad, and the Ugly…)

Point is, this is a timeless character archetype. You’ll find examples in basically every kind of storytelling.