The more people you love, the weaker you are. You’ll do things for them that you know you shouldn’t do. You’ll act the fool to make them happy, to keep them safe. Love no one but your children.” (Cersei Lannister)
These are the words of a person who knows about her weakness imo, the last line doesn't mean she's not weak cause she have love for her childrens only, it means the love for her children is her weakness.
I think, it's more an excuse on her part than a wisdom. She is emotionally damaged and has a huge inferiority complex.
Since Cersei is too weak to change herself, she settled to be extremely loyal towards some (Tywin, Jaime, her kids) and completely merciless to the rest. The "love" she speaks of is rather the last positive piece of her broken inner emotional cosmos. She combines it with her addiction for approval and reflects on the few people she is loyal to (the only ones she even considers worth living). Well. Except for Tywin, who she would
love, but who forbids all his children to show him love.
It's interesting. When Jaime told Cersei those "beautifully romantic" words about killing everyone until they are the only 2 persons left, he kinda reflected Cersei more than his own feelings. He's much more complex. But he's devoted to her and follows her screwed-up philosophy.
I don't think it's been addressed. Not sure it needs to until we get more BASTARDS. /spoiler?
I remember it was mentioned at least once, more likely twice. But with so many surrounding details, characters and side stories it's easy to miss.