I think that Cora loved someone once, not Henry though, rather Rumple and/or King George, and that kept her from obtaining absolute power in some way, which is why she believes it’s a mistake to love.
If she’d allowed herself to become completely infatuated with Rumple, and let her guard down with him, she was vulnerable to his tricks and manipulations. Being played by Rumple wouldn’t sit well with Ms Cora at all.
If she’d had an affair with George, the fallout from that could have led to Henry losing his kingdom, which is why they weren’t royalty when Regina was young.
I’ve got George pegged as being Regina’s father right now. I’m thinking that they were having an affair, and Cora was fed up with being the other woman, but George wouldn’t leave his wife, then Cora found out she was pregnant, and she’s the one who cursed George’s wife to be barren. Maybe she couldn’t have the man’s heart, but she could have his child, and she’d make sure his wife couldn’t do the same as a means of punishing him for not returning her love. Perhaps her machinations were discovered, and there was a threat of her being exposed, and perhaps she’d have been thrown in jail or something, unless she (and Henry) made some sort of deal where they got to keep their freedom, but they lost their royal status. Also wondering if Rumple’s castle used to belong to Henry and Cora, and he got it through a deal such as this.