[This is a comment that I made at Songs from the Wood that I think worth preserving for posterity.]
I almost want to say something similar, that I have never believed in God. I suspect that to say “I was always an atheist” is too reductive though.
For me, it feels more accurate to say that an unbelieving part of me has always struggled against the Mormon part of me. I always doubted, and as you say, never fully internalized all Mormon beliefs. The Mormon part tried to strangle the nonbeliever, but could never fully succeed. During the decades that the Mormon held sway, the infidel stealthily gathered strength. There came a day when the infidel rose up and sucker punched the Mormon, and the struggle ended.
I have peace of mind now because two parts of me no longer battle for supremacy. The Mormon believer no longer exists in any meaningful way inside me.
Looking back, I am tempted to project my single-mindedness backward in time and claim that the single-minded infidel was the authentic me. I hesitate because I suspect that I had an authentic experience as a Mormon, that many people who call themselves Mormon are having the same kind of experience.
By some particular definition of the word, I was Mormon for a while.