I don't feel betrayed, fortunately. My parents were also indoctrinated and taught us what they understood to be true.

However, when I found out they lied to me about Santa? I was royally pissed! They KNEW that was a lie, so I felt betrayed in that case. Once I found out the truth I was the most passive aggressive little shit for months.

"You're my real parents... I'm not adopted? Are you sure, or are you lying about that too??"

I was almost forty when I got out and yes I feel angry, betrayed and annoyed to have wasted so much time and money on a fantasy. I also feel profoundly lucky to have escaped when I did.

Possibly not as hard to imagine as you think. Although I grew up in the JW cult and was often persecuted as a child because of it, I had a neighbour who was an atheist in a family of atheists and he was equally persecuted for his lack of belief by the kids who were from homes which adopted the majority religions.
I have often wondered what it must have been like to grow up as a Catholic with all the ritual bull shit that goes along with most of the major religions.