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The day Regina Mills realized she was in love with Emma Swan was the day she learned that Rumplestiltskin's son was Henry's father.
Her first reaction to the news was shock; this was a development she never would have guessed. Her second reaction was the sharp sting of betrayal at having heard the news from the Imp rather than from Emma herself.
Regina realized that ever since her mother had framed her for the cricket's supposed murder, she and the sheriff hadn't exactly been on speaking terms. But hadn't, after all they'd been through, after raising Henry for ten years on her own, she earned the right to know that her son's father was back in the picture?
But even stronger than either the shock or the hurt was a surprising pang of jealousy. The thought of Emma having been intimate with Baelfire made her blood boil. How dare he have touched her like that? How dare he know her body in that way? What was his purpose in showing up now? Did he want her back? No sooner had the questions crossed Regina's mind, she was taken aback by the absurdity of them. Why should it matter to her who Emma had slept with eleven years ago?
Because she wanted her for herself; the realization hit her like a ton of bricks.
Somewhere over the several months that had passed since Emma had first arrived in Storybrooke, the Evil Queen had fallen in love with the Savior. Exactly when that moment had occurred, Regina couldn't be sure, all that she knew now was that at some point it had. If it had been anyone else other than her, she would have laughed at the irony.
As it was, all she could do was stare at the walls of the living room as she sat there, processing her feelings. Thinking back on things, the fact that she was in love with Emma explained so much. Such as why it had hurt so much when the other woman hadn't believed her when she swore up and down that she had nothing to do with Archie's murder. By then she should have been used to people doubting her word and one person's opinion shouldn't have carried so much weight. But it had and now Regina understood why.
If they had been different people, it would have now been the time for Regina to decide when to make the first move. But in this situation, there could be no first move. Not only had she been responsible for the curse that had torn Emma's family apart, but she had tried to poison her (only to wind up nearly killing their son instead). It would be impossible for Emma to ever return her feelings; she certainly wouldn't have been able to if their positions were reversed.
Unbidden, Regina's mind wandered back to the day Emma had saved her first from the angry mob and then from the wraith. "She's not dying," Emma had said and her heart had skipped a beat. She supposed that should have been the first indication of her feelings, but she had been too bound up in a mixture of fear and a need for self-preservation to notice. Had it, just maybe, been a sign that Emma cared for her too?
She dismissed the thought as quickly as it had come. No, it wasn't. Emma had acted purely out of love for Henry when she had save her; just as she had invited Regina to the welcome back party for his sake.
All she would ever be to Emma Swan was their son's other mother. Just as the love she had shared with Daniel had been doomed, so would be her love for Emma. Best that it never be pursued; that way her heart would never be broken.