She's forgiven Emma.
There is a lot to be said about casting curses, especially big, tedious, complex ones. The theory and forces behind one can, and have been debated by many great, magical minds through the ages. However, the thing with theory is that it requires no experience with practice, and one can debate the finer points of theory for great lengths of time and still not come close to touching on the finer points of practicality.
Regina knows this. When you live twenty-eight years in a town created by a curse you enacted, there is plenty of time to ponder not just the theory of the matter, but also the lesser known practical aspects and the repercussions.
A curse can be elaborately planned, and all the moving parts in order, but when push comes to shove, once the curse is set in motion, the details that can only be set at that point, that no one told you of, that you had no time to think of, are easy to get just a little less than perfect. There are a great deal of things Regina would have done differently in those split seconds if she knew then what she knew after a year in Storybrooke.
Emma Swan had no real aptitude for magic, a lacking interest, and no concept of how dangerous it could be, and unfortunately despite all of that she was still filled to bursting with magic, effortless and easy, where Regina had fought so hard for hers. Regina secretly willingly forgave her the glaring difficulty with casting spells so basic they could be compared to flipping a lightswitch; so in the end, she couldn't every well hold a grudge because she had some trouble with an enormous, complicated dark curse.
Can't blame Emma for trying so hard to do right by her when she can't remember the last time anyone else did.
She had still been barren, Doctor Whale had confirmed before Henry. The effects of the potion she'd taken in the Enchanted Forest had not been negated by the curse placing them in Maine, but pure, true love magic and an endlessly stubborn Emma Swan seemingly could circumvent anything.
Two years to the day after Emma Swan took her first stab at bringing about a curse even Rumplestiltskin balked at, Regina walks across neat grass, Emilia carried on her hip, coming to a halt by the stone reading Emma Swan and Henry Mills, colorful flowers blooming beneath their names.
She's forgiven Emma the curse. She's not sure she can ever quite forgive her leaving them all alone.