It is understandable why Norma had deep seated emotional issues, given her unfortunate childhood: her messed up parents, and the abusive, incestuous, continual rapes she endured. She was victimized and traumatized as a child, and didn't have a chance to form normal trusting relationships. She never saw a healthy role model for handling frustration, or for asserting herself in a mature, honest, adult manner.
In fact given what we know of her childhood and formative years, it is a minor miracle that Norma was able to interact with the world as well as she did. But knowing why she had problems doesn't alter the fact that she had them. As one lady put it, she was a train wreck. Her deep lack of trust in others, her fears of abandonment, and her unnatural and at times inappropriate relationship with Norman, left her almost completely without emotional coping skills or the ability to make smart, rational, decisions. She was never able to step back and look critically at her own behavior, or to look forward and foresee the obvious, looming disasters her behaviors either caused or exacerbated. Norma seemed unable or unwilling to take responsibility for her actions, and she was often dismissive of all but her own concerns. When Romero put himself in danger to help her try to leverage the flash-drive, and when he subsequently was shot and nearly killed, Norma dismissed the incident as insignificant, telling Romero the bullet went right through him, he wasn't dead, so no big deal. When her livelihood was endangered, or hers or Norman's safety were endangered however, she was almost hysterical, and frantically insisted that Romero act on her behalf. In some ways Norma was even more damaged than Norman. She was a stubborn liar, a defiant liar, an angry liar. She consistently lied to avoid having to face or acknowledge even fairly insignificant facts. When the situation forces her to finally admit the truth, Norma instantly switches gears. She is no longer the defiant, angry, victim. She instantly begins pleading, crying, and manipulating. As soon as she gets what she wants she reverts to her self centered posture, her denial and avoidance behavior, her facade. Poor Norman. He never had a healthy role model. He never had a chance. Norma constantly traumatized him, exacerbated, and worsened his psychosis.

Vera Farmiga does a great job portraying this damaged, unstable woman.

Norma lived in denial, especially concerning Norman. She lied to herself and everyone else about Norman's psychological issues, his blackouts, and his strange behavior.
Norman gradually learned to deny and lie like his mother. In the first season Norman was pretty honest overall, but he had other more serious issues, e.g. his blackouts. When Norman did lie it was usually because he had to cover for his mother, or he lied to Norma in order to try to avoid Norma's irrational or overblown reactions or emotional meltdowns.

Most of Norma's problems could have been avoided if she had ever learned to face the truth and been more honest with others. Had she been honest with the psychiatrist about Norman's behavior he would not have come home and been able to kill her as he did. Had she called the police when she had to kill the previous motel owner in self defense she could have avoided all the lies and trouble with officer Zack Shelby. The list of examples throughout the series seems endless.

Once in the first season she was honest with Norman about his mental problem; hearing and seeing things which were not there, telling him that he did so when he went into a trance. But in later seasons she covered up and denied to herself and to Norman that he had any serious problem. She dismissed the idea that there was anything seriously wrong with him for as long as she could, and never did fully admit it to herself or anyone else. Occasionally someone, Romero or Dylan, convinced her to seek appropriate help for Norman, but she would slip back into denial and allow him to quit therapy. Her inability to cope, her denial, and her self deception eventually cost her her life.

I

One of the most human moments of the entire series happened in Season 3 E7 , The Last Supper, when her brother Caleb showed up unexpectedly to the house to leave some flowers and a note for Norma. Seeing how utterly broken and remorseful he was, Norma invited him to stay that evening for supper. And for the next hour or so Norma was able to escape her demons. She and Caleb sat at the piano and sang togeth one of the old songs they sang as children, presumably in the happier times before the sexual abuse began. Earlier Norma had helped Romero when he was terribly drunk, caring for him gently and compassionately. Romero wound up joining the family for dinner. Emma was also there. And at that dinner it seemed that Norma and her loved ones were the happier than any time in the entire series, exercising forgiveness and compassion for each other, free for those moments of any stresses. I think Romero began to fall in love with Norma that day. Even Norman was relatively stable and able to tolerate having to share his mother with others that evening.

I was really surprised Romero could stand Norma given her nearly constant dissembling. Romero didn't put up with people lying to him. And Norma lied to him almost continually.