Speaking from personal experience ( and observing some others) I can say that I believed I was in "The Truth" because I WANTED to believe it and wanted to believe the things taught. I'd made a real mess of my life and believed that being a JW was the way out of the mess and to solve all the problems in my life. So, you could say that my motivation was not to serve God but rather to get out of life what I wanted. Selfish, huh?
And so I guess God let me believe what I wanted to believe - including the lies, or to be more charitable the untruths. (I don't think the GB or others within the org set out deliberately to tell lies).
I'd also like to say that it seems to me that many former JW's have totally abandoned belief in God and faith in the ransom of Christ because of finding out the truth about "the truth." This in my opinion - no it's more than just that - it's based on the truth in the Bible - is a huge error. God still exists even thought the GB have got many things wrong - and Jesus Christ is the only way by which someone can be saved.
Finally, although the GB are proven scripturally to be wrong I personally made some good friends during my time as a JW - and did learn some valuable spiritual truths. And I try every day to grow closer to God whom I now know to be according to 1 Timothy 3 v 15, 16 as Jehovah who came in the flesh as Jesus Christ.
May God Bless all