This was great.

I agree with one of the reviewers that it could have been flushed out, but it makes sense to me that they would react in a passionate way. I've been there before.

Aaahh this hurt but was so beautiful too!

Nothing to say about your way of writing, and the first half of the story wasn't bad but then it all went downhill. It literally couldn't have been more unrealistic!
In a span of 5 seconds Regina went from being a heartbroken woman to acting like a horny teenager ("her calves hitch up on his thighs" c'm on, seriously? Just because he said it was a lie? No further explanation needed?).
Look, I'm not trying to be mean or anything, trust me. But next time you write a fanfiction (be it a one-shot or a multi-chapter, a drabble or whatever), take your time writing it. Put yourself in the characters' shoes. If you were Regina and your soulmate had just told you that he had gotten your evil sister (disguised as his wife) pregnant, therefore implying they had sexual intercourse while you kept looking for your happy ending, never giving up hope that said happy ending would include you two ending up together, would you really have gotten over the heartbreak so easily?
He just says 'it was all a lie' and she doesn't even bother questioning it before she lets him stick him tongue down her throat, all mere minutes after he has dropped this huge bombshell on her?
Emotions can't be turned off and on like a light switch, no one does that, it's not that easy and if you write that it inevitably comes off as awfully unrealistic.
So please, next time don't rush through such a delicate situation. Instead, take your time describing what's going on and how the situation evolves (e.g. Robin said it was a lie, but what was the lie? The fact that she was pregnant? The fact that they had slept together? And why would he say that if he wasn't true? Did Zelena put him up to it? did she threaten to hurt Roland?). Don't just scratch the surface, go deeper.
I hope I haven't been too harsh but that I've made my point and that this review will help you be more critical towards your own work in the future.

I Loved it. As an OutlawQueen fan, I'm wishing this was the truth. Awesome writing.