Be aware of the things that a protagonist admires about himself that are provably false. Then prove it.
So, me. I would never change my dynamic, awesome female protagonist into a man. I worked hard on that character, it’s important to the story that she’s a woman, it’s a groundbreaking part an actress would kill for, it’s who I am and it’s what I stand for in my work and in my whole life and changing that character would violate my principles and how I approve of myself as a person, a woman and a writer. I will shop it around until I find the right home for the project.
Now. Offer me this.
No! I will not compromise those principles for the money. What? Go to hell.
How about…
Christian Bale, circa 2000, wants to read it after the rewrite.
Well. That’s interesting. Would he be in the shower? What am I thinking? I’m not changing my protagonist. It goes against everything I believe in.
Okay. One time special offer…
Jennifer Lawrence playing me in a movie based on true events in which I win my Oscar for a script about a dynamic, awesome male protagonist.
I just sold out. I actually struck a blow for the patriarchy. I’m a monster.
Choices that challenge your protagonist’s beliefs about themselves reveal the character in ways they might not be very happy to contemplate.
That is a fun tool for your toolbox. Pick a critical moment, go to the crossroads and make your protag a deal.