If I Don't Do It, Who Will? Transitioning From Daughter To Caregiver

At what point as a caregiver, does the realization hit that we must set aside our lifelong role of daughter or son, to become a caregiver to our parent? How hard is that transition to make? There was no one else who would step in to do this, so if I didn't, well, the alternative was not even a consideration for me.