Why did Marika turn selfish?

I have recently just started Elden Ring because I got it as a christmas gift from my uncle and have loved literally every second of it, the game has some insane lore that I wanted to get into and I decided to start with Queen Marika's story and try to understand why she shattered the Elden Ring, however one thing that I don't understand was her reasoning for turning selfish from a being of hope and deciding that she would fight the greater will, can somebody try to explain both speculative and non-speculative info to me on why she did this?