What should I do about growing up in an environment where marital infidelity has become commonplace and my mother's psyche has been destroyed due to my father's bad behavior?

The default is to assume the best of a Muslim and not accuse them of anything disgraceful without clear proof, especially concerning a father. If it is proven that the father committed indecency, then the duty is to advise him gently and kindly. If he persists, it is recommended for the mother to seek a divorce due to his transgression, and she is not obligated to obey her parents if they forbid her from doing so. Good people should cooperate to bring about reform, and whoever fears weakness for themselves should leave such an environment for a righteous one.