Doctrine. As a born-in I completely believed everything & thought everything I disagreed with e.g. subjection of women, Cedar Point, Ohio, death of Solomon's baby etc was my fault for not having a good enough understanding. Then over the years my "wait on Jehovah" mental folder just became too big, & it crushed my faith.
As soon as I started trusting my own judgement the doctrines became laughable.
Many believing lurkers will now be thinking of the scripture "Do not lean upon your own understanding", to which I'd just say that if you're going to submit your whole life to someone elses "understanding", you'd better make damn sure they're worthy of your trust.