Yes, I did.
I was my mother's primary caregiver for the last 4 years & 2 months of her life. I was with her every night for the last 2 weeks of her life. I was one of 2 people who's signatures authorized the removal of the vent, the discontinuance of meds (other than morphine & Ativan for comfort care), and I was the last family member to see her at the hospital before she was transferred to the funeral home. So, yes, I'd say that qualifies me to speak on this.
It's a hell of a thing to have to go through - the second-guessing, the waves of guilt, the feeling that you're taking a human life. Been there, done that.
What I can say is that, with time, it will pass. As others have mentioned (and I'll spin a little differently) you're acting in accordance with the family's wishes, and those wishes are in accordance with State and Federal law. It's distasteful, it's heart-wrenching...and it's all perfectly legal. And, ultimately - it's right; because it's what the patient wants. It's their choice to make, not yours. To do any differently would not only be illegal, but frankly is over-the-top disrespectful. You don't have the moral right to second-guess a decision of this magnitude, and it's not your decision to make - only the patient and their duly appointed representatives have that right. You're there for the sole purpose of providing care according to their wishes - that's it.
Other than requesting reassignment in future instances, I don't have any professional recommendations to make. What I would do (and, what you're in fact doing right now) is to sit down with a favorite adult beverage & consider the circumstances where death might be preferable to life. There's quite a few if you think about it - MCS, PVS, anencephaly, strokes, various diseases (ALS for one); do you really want to be the one to be doing extreme life-saving measures that could be interpreted as medical torture? It works both ways, as most things in life do.
No, it's not an easy thing to face - but, you'll be a better nurse for it; and a better human being as well.
Welcome to the world of the survivors. Peace be unto you.