I never remember having any doubts until a few years ago. I was a true believer, although I do remember feeling really uneasy whenever I'd hear someone had become an "apostate". I did wonder if they knew something I didn't, and then I would chase that thought out of my mind.
I don't know why it wasn't until my 30s that I started noticing the inconsistencies, in the Bible and in the WT publications, but once I started noticing, it was hard to shut off. I shoved those doubts aside for another couple of years. I turned to super intense study of WT publications and bible reading to reconcile my doubts, and that's when it all fell apart very quickly.
Things that had bothered me: disfellowshipping arrangement, secret judicial hearing, the arbitrariness of the decisions (I realized that if you could appeal, then their judgment could be wrong the 1st time, yet we weren't allowed to ever question the elders' decisions), "encouraging" young teenagers to baptism, Old Testament's Yave orders to kill women and children and to not feel sorry, treatment of women like 2nd class citizens, Paul's writings and the inconsistencies, the dogmatic statements made in the WT, etc. etc. etc. etc. etc. And, then when I started reading about "apostacy" in the WT, I realized something was really wrong - it really sealed it for me.
When I approached my husband, who was an elder, about my concerns that the GB was self appointed and that there was absolutely nothing to back up their claim, he tried to "help" me. But a few weeks later, his own doubts after having been an elder and observed that the a**hole with the strongest personality, if he happens to be the P.O., makes the decisions, and that there was never any evidence of god's spirit on these meetings or decisions.