I had some concerns about this story, but I was kind and suggested contacting social services.
And there are several reasons why:
1. When you first get into something, you tend to have a missionary zeal. For example: when my fountain pen obsession started, I went a little nuts. When I started preaching, I got serious about reading and sharing the Bible (though ironically, that hastened my atheism). When I first became atheist, I wanted to debunk everyone.
2. Younger people, especially teenagers, don't always explain things the way those of us who are adults would. I quite clearly recall my second grade teacher slapping me and knocking me to the ground. At this point in my life, it's nothing. At the time, it was a pretty big deal, and I suspect that my memories make more of it than it actually was. So, there could easily be truth in this story. But, for example, a hand on the cheek to get attention could easily be interpreted as a slap. There could easily be more to why this person was institutionalized than atheism. For example, some people (especially kids) are still institutionalized for being homosexual. They can also be institutionalized for self-destructive behavior.
3. Stories change to make the subject look good. Apparently I'm terrifying when I yell. That's what my students tell me. Now, I'm not a yeller, so I'll concede that I have the element of surprise on my side. I broke up a fight at the school where I teach two years ago and was told how scary I was. Now, the reality is that I got their attention, but I was pretty calm. But, it was their way of saving their manhood.
4. I can believe that the OP is struggling with religion. Sadly, I do know that some families are horrific. One of my colleagues apparently believes I'm given over to Satan. Being a 40 year old man who can calmly say, "What are you going to do about it?" in response to her hysteria is a different place from being a minor child, especially a minor child who is outspoken.
I suspect that there is something going on with the OP. Whether the account reported is entirely accurate is a different question.