Joanna/Tywin as a framework for Cersei/Jaime
All children are shaped, after some fashion, by their parents' relationship. Joanna died when the twins were around 7, according to math, so in many ways, their "parents' relationship" as they observed it was really more Tywin's relationship with the memory of Joanna.
To fall in love is to create a religion that has a fallible god.Jorge Luis Borges
The trope of the highly idealized lost love is a common one. Without having to contend with the imperfect reality of a flawed human being, or the grind of daily life in a long-term relationship, there is nothing to curtail these inflated ideas of one's partner.
The topic of idealizing one's partner is present in all relationships, to one extent or another. And yet Jaime and Cersei's relationship still stands out to me as unusual in this regard. I cannot think of another example where people idealize a living partner in a long-term relationship to this same extent.
Love is an ideal thing, marriage a real thing; a confusion of the real with the ideal never goes unpunished.Johann Wolfgang von Goethe
This idealization of each other is one of Jaime and Cersei's core relationship problems. They grew up seeing marriage as putting your partner on a pedestal. That is what was modeled for them; that's what they were taught that love looks like. But while this might be a reasonable way to honor the memory of your dead wife, it's not a healthy way to view your living partner.
(Oh Tywin! Even when you're doing sweet things like expressing your love for your beloved wife, you still manage to mess up your kids!)
This framework of idealization is also why I'm on board with the "Jaime had a crush on Arthur Dayne" headcanon. The way he talks about Arthur mirrors the way he talks about Cersei. He's a fanboy; he thinks about Arthur like his sempai who noticed him.
And because Arthur—like Joanna—is dead, there are no real apparent consequences from this mindset. But no real human, present in one's life for a significant length of time, can live up to that. And that disappointment, that disenchantment, is the source of so many of the twins' problems.