Inspired by @muaka-safari's analysis of Vakama's relation to anger.
My hypothesis: Nokama, while stressed too about fulfilling her duty, has a different core issue in relation to it. Rather than the aspect of fulfilling a command by her leader, she constantly thinks of her behavior. For her, her conduct is part of her duty.
She has to act a certain way. She has to be in a certain way, or she is failing.
Which makes sense. She is a teacher, after all. She has to be a role model for her students. But what if it went deeper than that?
Nokama puts a lot of stock in order, reflecting her Metru and the vahki assigned to this metru: The bordakh. These vahki carry staffs of loyalty that instill uncompromising desire for social order.
Official material depicting in-universe information describes Ga-Metru in a very flattering light. It's the city's spiritual center and said to be favored by Mata Nui himself.
None of the Ga-Metru hold any jobs in terms of repair, manifacturing, or engineering. Matoran from other Metru do these jobs for them. Ga-Metru is privileged.
But like with everything in Metru Nui, this is only one side. The side most would think about first, whether as good or bad. The other one of this reputation means that all ga-matoran must be peaceful, right? They are orderly, spiritual, wise, etc. Otherwise Mata Nui wouldn't love their Metru as much.
Ooof. Of course, some would feel arrogant about that. But mostly, that'd be a lot of pressure. If you're the favorite of the city's most important person - your God - then you have to show it that you're showing reasons why you're the favorite? And keep it up all the time?
I doubt all ga-matoran took it that deeply to heart, but some of them likely did. They have to act responsibly. Among them is Nokama.
Remember what I said farther up in the post that she thinks she has to act in and be a certain way? We see it regularly.
Nokama in the movie and in the movie's novelization [[gave the Po-Matoran a sharp look. “Your negativity pollutes this sanctuary, builder.”]]
She snaps at Matau and tells him they have to take their duties as toa seriously.
She scolds herself after she argues with Vakama in the Archives. Never mind that it's a genuinely stressful situation and the others barely ever consider that they themselves need to make an effort to argue less. In my reading it felt like she deemed it more a moral failing of hers rather than a practical concern?
She tries to mentor Vakama and steer him into the direction of being leader. We're never given a concrete in-universe reason. (Out-of-universe, the writers clearly pushed the fire toa into the leader role.) But maybe Nokama saw it as her duty and responsibility to keep teaching in the one way she could, if she wouldn't teach matoran anymore? (Not to mention that's the only connection to her old life that she still has.)
She asserts that she's a toa in the comics when Nidhiki mocks her, Matau, and Vakama to be overgrown matoran.
Feeling rejected by the Great Temple when she enters it as Toa Hordika deals a heavy blow to her. She feels impure, as if she's longer fit as she is to serve the Great Spirit in his regard.
She has a rage breakdown, yelling that "She's still a toa!"
On the other end, Nokama notably has moments as Toa Hordika, while she's trying to calm down and find stability with Gaaki's help, where she doesn't seem to care about her previous high standards anymore. She feels tempted to give into her hordika side and is much more reckless, less thinking about how she acts as a toa.
She also makes an effective and chaotic team with Matau. As @crystaltoa pointed it out, they both are now "hold my beer" types, while they act about each other as "I can't take you anywhere with me!"
As if the transformation into a hordika beast, which looks nothing like a toa's supposed to, has freed Nokama from a great weight.
(@crystaltoa She might still think back on the Great Temple, however, and wonder if the stain on her from being transformed into a Toa Hordika remains.)