It's kind of comparable to how Tywin Lannister doesn't want Jaime to enter the King's Guard - despite it being an enormous honour - because he thinks that as a Lannister, Jaime is too good for that, and he wants him as his heir. I'm not sure what Jaime intends to do about Cersei, but I certainly think he doesn't love her anymore.

And as for Cersei, it is disappointing just how easily she goes from powerhungry and paranoid to batshit insane, but I don't think Martin means to imply that she was crazy all along. She starts out merely overambitious and strongly defensive of her children, and somewhat jealous of men and resentful of the role she's pushed into as a woman - partially because of her extreme closeness with Jaime who does get to fight and all, no doubt. Then as she gets more power, she gets overambitious, with a hugely inflated idea of her own abilities, and starts making bad mistakes. As she finds out about those one by one, she gets increasingly paranoid and convinced that it's not her fault somehow. In the meantime, her jealousy of men increases until she basically becomes a poster child of penis envy (as the lesbian sex scene painfully obviously illustrates). So the whole evolution makes sense, really, it just seems too fast and too easy.