They WILL regret ending it IF
They will regret ending it if you were a kind, caring, thoughtful, considerate and loving partner. They'll regret ending it if you truly loved them unconditionally and they felt the same.. If you have done and would've done anything for them.. If you gave them everything they ever wanted and would've given them the world.. If you did everything in your power to make them happy.. to hear them laugh and see them smile.. that same laugh and smile that once brightened even your darkest days.. If you were always there for them, always cared so much about them and showed them.. If you were their shoulder to cry on, listened to their every thought and complaint and always tried helping as best as you could.. If you cherished very moment you spent together and you were good to them even when they weren't necessarily good to you.. then they will regret it. They'll regret losing the person that put them on top of everything and everyone else.. the person that valued them more than life itself.. the person who accepted every single thing about them and would never judge them. They'll regret throwing away that kind of love especially if they're at fault, because to you, they were it, and not long ago they felt the same. They were your light in the dark.
You weren't perfect, of course, everyone makes mistakes and no one is perfect (as I write this I still think that EX is perfect even after EX hurt me and I've had a lot of time to think it through), but if you were your best self and tried to be perfect for them, and you showed them that you put so much effort into them and the relationship, then they will regret it. They'll regret it especially if they admitted all of those things above, and they felt the same way about you not long ago, and that you didn't deserve it and that everything was their fault.. because it doesn't change the fact that you're now the one suffering and they hurt you, when all you ever gave them was every part of you.
It may take weeks, months, or maybe even years. It may take for them to get hurt by someone who doesn't value them as much as you did, or someone who isn't as kind and thoughtful as you were. Or maybe one day they just realize what they threw away. Either way, one day they will regret throwing away everything you had.
Edit: I don't know everyone's situation or how their ex partners/relationship was, just trying to put some hope out there :)