I suppose Regina had a moment of weakness when Rumpel stopped her and tempted her with magic. With no where else to go, so probably figured she might as well marry King Leopold and learn magic from Rumpel on the side. It’s quite ironic, because she ended up fulfilling her mother’s wishes, despite having pushed Cora through the looking glass. She even turned into her mother. I’m actually looking forward to the mother-daughter reunion between Cora and Regina, especially now that Regina is struggling to give up magic. It seems like she’ll have to resort to magic though to keep Cora and her powers at bay. A recovering magic junkie should not be around a fellow magic junkie if she wishes to recover.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy