Reddit user Rain 12913 claims to be a psychologist and offered analysis and diagnosis for the character Cersei Lannister on Game of Thrones. She's a fictional character, which the psychologist was quick to point out. But it's fun to use a real life diagnosis to explain the driving motivations of Cersei. It turns out she has a personality disorder. The fact that she cares mostly about herself—or people with personalities that are nearly identical to hers—makes her a classic narcissist. As always, there are a whole bunch of spoilers for the series ahead.
To be certain, Cersei would have plenty of certifiable mental health issues. Sleeping with your brother would be a paraphilic disorder, and a penchant for wild fire and pyromania would be classified under conduct disorder. But here's an analysis of her emotional responses and actions that make her a narcissist:
Cersei is a classic narcissist. As such, she lacks the ability to truly empathize with others. Despite this obvious reality, people seem to be falling into the trap of thinking that Cersei really does genuinely love her brother and her (late) children. While she certainly says that she does quite a bit, and while her behavior may seem to suggest that she does, it is highly unlikely that such a narcissistic character is capable of true love.
It did seem that Cersei genuinely loved her children. She was devastated over the death of Joffrey, and he sucked the most. But there's an explanation for what appears to be a narcissist's love:
Rather than loving this other person as a separate entity who has their own strengths and weaknesses, the narcissistic person splits them into the "perfect" category, and considers them to be an extension of him/herself. You see this in the way that Cersei thinks about Jamie and her children.
There it is. She loves them only in as much as they act like and feel like a part of her. So it makes sense she wasn't completely losing her mind after Tommen jumped out of a window. He had already gone full boy scout when he began taking advice only from the High Sept.
How much will she pretend to love Jamie next season? Considering his attitude towards royalty that uses wild fire against people, they might have a few bumps in their relationship.