Willpower can be used to suppress emotion and act in a particular way. This can be useful but isn’t an effective long term strategy. Willpower is finite and sometimes fickle, in part because of the physical reasons you describe.
For most stimuli, our strongest emotional reactions are to our thoughts about the stimulus, rather than the stimulus itself.
A better application of willpower is to reject and replace the thoughts that lead to those emotions. Over time those thoughts are replaced entirely and the emotional reaction is changed.
Stoicism: dichotomy of control; Buddhism: tale of two arrows; Socrates: "The unexamined life is not worth living"; I'm sure there's more...
Humanity has produced a great deal of knowledge on how to live well. Modern society is just too distracted to learn about it.
A change in mindset must happen, but the proper mindset in which to change one's mindset is elusive. Even if my mindset today is flawed, what specifically should stay and what should go to make myself a better person? It feels like leaping from a safe harbor into the unknown. Can you convince a person to kill themselves and let a near-copy-but-not-quite live their life instead?
That being said, I think some positive change can be produced with diligence and care, even if the methods and details are hazy even to the person enacting them.