I don’t read spoilers so this is just based on my own speculation —- I get the sense that Regina’s pain is somehow to do with a child. In “Snow Falls” Charming asks Snow if she really did ruin the Queen’s life and Snow says “yes.” I don’t think Snow stole EQ’s boyfriend. Snow seemed almost regretful when she said yes, and I don’t see that as a rivals for love relationship. I also see EQ as a generation older since she was married to Snow’s father. I don’t have a scenario of how it might have happened but I am thinking maybe EQ lost a baby or lost the ability to have a child. That would be consistent with Regina wanting Henry.
Just a theory.