In the whole tv series we are always told that
true loves kiss breaks all magic. Now in one flash back in fairytale land we see that Rumplestiltskin and Cora had somewhat of an affair and Rumple taught her how to use magic. He fell in love with Cora and she fell in love with him.
She explicitly states that she removed her heart because it was stopping her because she loved him.
My question is why is that their kissing did not break the Dark One's magic but Bell's kiss was breaking it? Both were true love's kisses why would one work and the other would not?
Also please use spoiler tags if you are going to refer to a specific episode or specific moment