A headstrong protagonist- provided they aren't an antihero- should have just enough ego to rub an authoritarian the wrong way, but not so much that they become completely unlikable and ego becomes their defining trait. If you're writing an antihero, then you can make them as pathologically narcissistic as you want but you don't want to turn them into a caricature if you're going for realistic traits.

A protagonist made of milder stuff shouldn't be so meek that they lose themselves in the face of a strong personality. Wishy-washy heroes and heroines who don't have hard limits on how much abuse they will take become a cheap device for pity and don't differentiate themselves as individuals enough to care about. They might also be unaware that they are being abused at first, or at least they don't articulate that awareness very openly and clearly. Righteous indignation makes for compelling characters even if they don't go full on Batman when they've been pushed too far. Sometimes, a simple "no" where the reader has come to expect a "yes" makes all the difference. A mild protagonist is almost never an anti-hero as they are a natural underdog.

A protagonist of average temperament should be torn about how much abuse they should take, as they will know without a doubt that they are being abused but are reluctant to rock the boat unless their limits are crossed. Their limits should be less extreme than the limits of a mild protagonist. For example, where a mild protagonist might draw the line at losing their children but not be very concerned with having their autonomy, and a headstrong protagonist might be extremely hawkish about their autonomy, an average protagonist will tolerate some loss of autonomy but draws the line when certain decisions are no longer theirs to make. An average protagonist is usually made into an antihero by having quaint beliefs, a streak of stubbornness or stupidity, or simply being casually crafty without necessarily using that craftiness in a vindictive or calculated way; that's more the domain of a headstrong antihero.

Another thought I've heard- and I find this to be broadly true of all developed characters- is that your protagonist should have a real personality. Try describing your protagonist without using their gender, species, age, occupation, nationality, race, etc. If you can't, then what you have is a superficial character. These are fine if they only occupy bit parts, but one of the things I hate is when primary characters are only defined by their superficial traits. One of my biggest complaints about Hollywood is their tendency to define a character only by those things that a census taker might be interested in (the Star Wars prequels were really bad about this). You also see it in bad fiction, especially in today's "insertion fantasy" fiction where a protagonist is written as a blank template for the reader to project their own identity onto (Twilight comes to mind).

It isn't that hard to write a protagonist. Just make sure they act and think like real people.