We know that she told Henry she loved him for the first time, kissed him for the first time, and the curse broke right at that moment. Emma was desperate and heartbroken and thought she had lost him forever, so maybe that was a factor. Henry also loved her back, as we saw when he awakened, so that could have been a factor too.
We don't know the rules of the curse yet, or how this met the requirements for the curse. We don't even know for sure if Emma had to "beat" Regina to break the curse.
This is my theory about why the curse broke in this way, and I'd love to hear your theories!
I feel the break was multifaceted that it could be interpreted in many different ways. If Emma had to “beat” Regina in order for the curse to break (the final battle), I think it was broken in three ways; by Emma loving Henry, by Henry loving Emma, and by the Power of True Love.
- - Emma saying I love you (likely the very first time she’s ever said this), or loving anyone, broke the curse. She “beat” Regina because Regina indirectly took away Emma’s happiness, humanity, and ability to love when she separated her from her parents, dooming her to be a neglected orphan. Emma being able to overcome 28 years of neglect and abuse, and finally love and accept others into her life, was her beating Regina and it broke the curse.
- Henry loved Emma, and directly chose her over Regina. Even though Regina had 10 years with him and no obstacles in her way, she couldn’t make him love her (I’m assuming due to the void), or show him the kind of love Emma could in just a few weeks/months. The fact that Henry was damaged his whole life, and then was able to love damaged Emma despite everything Regina did to stand in the way, was “beating” her and broke the curse.
- The Power of True Love between Emma and Henry broke the curse. In a way, Regina separated Henry and Emma in the first place, because indirectly her actions made Emma feel worthless and unlovable (see above), and therefore an unfit mother. The Power of True Love (Magic) “beat” Regina because Emma and Henry were magically able to overcome Regina’s dark magic and find their way back to one another. Regina’s power paled in comparison to the greatest magic of all, true love, breaking the curse.