Well hello fellow exmo. I gave it up in my late teens. Found myself playing "devil's advocate" too much in discussions with my friends. Tried to pray about it all Joseph Smith style, but just got absolutely nothing. Realized that I had never enjoyed Church, never felt at peace there, and just generally came to the conclusion that the essential problem of free will and comparative religion and the extremely specific truth claims that Mormonism requires weren't holding up. I was also completely eeshed out by the thought of a patriarchal blessing, and I felt no calling whatsoever to go on a mission. I wasn't as traumatized as some, growing up in the Mormon hinterlands of the American south (NE Florida) meant the LDS were a little less high and mighty and I had a circle outside of the church, but the pressure to conform and stay is very real.
I only resigned formally when my mom sicced the missionaries on my never-Mo wife and me after I moved to Texas.
Ultimately, even as religions go, its theology is very silly and its most ardent adherents are real jerks.