You talk about Kaladin’s views about lighteyes as if he was born hating them. He wasn’t. In his flashbacks, he regarded lighteyes as heroic, courageous people. His best friend when he was young was a lighteyes.
But then, ‘it’ hits the fan. Roshone starved his family, married his best friend and conscripted his brother to his death. Amaram, a lighteyes who was meant to be honourable and have never lied, killed his squad and sold him to slavery.
Then, another lighteyes, Sadeas, shows no regard for darkeyed life and has Kaladin strung up in a highstorm.
At this point, Kaladin has barely even had a conversation with a lighteyes who didn’t harm or kill him or his companions.
And you question why he finds it hard to let go of his beliefs? Old habits die hard, especially when those habits are forged by extreme oppression and circumstances.
It’s pretty much the same with Shallan.
Mental health cannot be pushed aside and dealt with later, in real life and on Roshar. That is one of the main themes of the series