I can think of a comparable situation. My brother was the father who earned the money. His partner (they never married) stayed at home, lived a lifestyle available to only the top 1%. They had a child.
They split, he generously gave his ex partner half of the assets. He had 2 more children.
There is nothing on God's earth that would persuade him (and rightly so) to leave to his children anything other than a 3 way split in his will, even though his first child will inherit significantly more than his younger two, because that child will also inherit from their mother, who did not earn the money. He realises that his first child would see it as an emotional statement even if he were to explain to her "I'm leaving you a bit less because you will inherit from your mother too, whereas your younger siblings will only inherit from me".
Arguably it could be galling for him because he has a special skill set and was able to earn significant sums of money, whereas his partner did not work at all: did not do any paid work in her life after the age of 28.
But he is not a bitter person and, as I said, he realises the emotional impact his trying to split the money evenly would have on his first child.
Your mother is bitter, emotionally unintelligent. Even if she perceives that her splitting her legacy 3 ways is unfair financially, she should have still done it, because of the emotional impact it would have on you.
It is not your fault @Eatcabbage. You've been dealt a rough hand by having her as your mother. Try to distance yourself and keep reaffirming to yourself that you have been unlucky. It is not you, it's her.