Hi, I'm Al.
My extended family are still all members. My wife and I left orthodox Mormonism in 1996. I was a mormon.
I had virtually no doubts about the LDS faith when I was growing up. Most non-LDS would say that is because I was "born in the church" and therefore I wasn't looking at things with an open perspective. On the contrary, I studied the religious beliefs and differences of various Christian sects while growing up. Along with several friends, I had several "debates" with ministers and the like from other faiths while in high school. My discussions with these people did nothing but further solidify my belief in the LDS gospel. The main reason for this was the information being provided by those "anti-Mormons". They would throw in some truth with many lies. They would also try and "prove" Mormonism false by comparing it with Christianity. I had no problem with the deferences between the two, and hence their arguments did nothing to convince me of the errors in Mormonism. Both mainstream Christianity and Mormonism suffer from the same truth finding methodology problem, but I didn't realize this at the time.
Why I left More answers about 'Why I left' the mormon church
The first time I personally ever had doubts was when I went to the temple for the first time in the 1980s. The whole concept of the temple was a great thing I thought at the time. Here I could be sealed to my family forever. Here I could help others who never had a chance to receive "ordinances" necessary for salvation receive them. When I actually went through to get my own endowment however, I was horrified by what went on, but I didn't say anything--similar to many Mormons. Not only was there nothing spiritual about the experience, the way the endowment is presented smells of cultism. You are instructed early on in the endowment that you can withdrawal rather than go through the ceremony. Of course no one withdrawals because you have no idea what is going to happen, you have your family and friends all sitting around you. Nothing of substance has happened yet to incline anyone to withdrawal. The next thing you know, you have taken a series of vows in unison with everyone else which hardly resemble anything you normally do in your LDS experience. Before 1990, you also had extreme penalties associated with the violation of any of these vows.
My wishful thinking got the better of me, and I continued to attend the temple--thinking that it was only my lack of understanding that caused me severe discomfort every time I went through a session. Then I saw a show on the masons, and I realized that Joseph Smith did a little more than invent the endowment on his own or somehow receive a "revelation" that enabled him to establish the endowment. I saw one explanation at the time claiming that he told others that he was restoring the corrupted portions of the endowment that had been lost and that the masons traced their ceremonies all the way back to the Temple of Solomon. I bought the explanation (hook, line, and sinker) at the time as I was a good little Mormon boy who would be going on a mission soon. Recently I found out this couldn't possibly be true based on my research of Freemasonry. In addition, many of the newer parts of the Masonic rituals (those created after the 16th Century) were incorporated into the endowment which shows that Joseph Smith borrowed rather than "restored" the temple endowment.
On my mission, several other doubts arose in me which I won't go into here. I did a similar cover up of each of those doubts like I did with the temple endowment explanation and tried to forget them. Then in 1990, the temple endowment changed. At first, I thought this was a great thing. I figured the experience would be more spiritual and less of a pain to go through. The more I went through the new endowment sessions, however, the more I realized that the changes made seemed more like an effort of the church to become politically correct rather than do things based on "revelation".
After getting married and graduating from BYU, I planned to just ignore my doubts and try not to raise any new ones to avoid any possible confrontations or conflicts with my wife. I should say at this point that I still had a good testimony during this period regardless of the doubts. It just wasn't a "perfect" testimony as it once was or seemed to be. I still didn't realize that "testimony" and the means by which Mormon testimonies are developed are very poor methods of discovering truth. I thought my wife would probably divorce me if she knew I had doubts so I never talked about them with her.
As time went on I felt I was living in a lie. I had to have my doubts resolved in order to obtain a stronger testimony. My first kid was on the way, and I didn't want to be a hypocrite in teaching him like I sometimes felt when teaching or giving a talk in the church.
I started doing some research into the temple endowment and the Book of Abraham--my two biggest problems with the Mormon church at the time. I won't go into any detail here in describing my ultimate findings and conclusions. Suffice it to say that after doing a great deal of research, I no longer had any doubts. The church could not possibly say that these two portions of the "gospel" are true and be true itself. There are many other doctrines I found to be untrue and many doctrines which I have found to have "evolved" over time rather than being "restored" as the church claims.
I don't want this brief history of myself to sound like I believe the Mormon church is completely 'evil' or somehow worse than other religious institutions. Some people may live happier and productive lives by being active participants in their faith than would otherwise be the case. However, the LDS church should not purport to be something that it is not--thus deceiving its own members. The church is a product of evolution and wishful thinking like the rest of Christianity. It is not based on revelation and restoration.