My supporting character is way more interesting than my protagonist

I'm working on my western for over a year now. This supporting character wasn't in the original idea, he popped up maybe when I was 3 or 4 months into the development. But now he's got too much impact on the story and I find him too good to let him go.

The whole story is built around the protagonist's past, struggles and character arc, but this character is way more interesting, and is stealing the show.

Anybody had the same "problem" (if it's a problem at all)? Or am I overthinking it? What could be the solution?