Thank you for your concern. I am of course unhappy to hear that my life choices have caused my family members to experience pain and grief. However, I must live a life that is true to my own beliefs and convictions. Simply put, I have realized for myself that the church is not true and it is not what it claims to be. This realization came from several years of intense study and deep introspection. I will not go into detail to describe to you how I reached that conclusion because I don't want to be accused of trying to hurt you or "lead you astray." But let me be clear that I am quite certain for myself that the church is not true. 
Again, it upsets me to hear that my life decisions make other people upset. I do not like to cause other people to suffer. But I cannot live a false life just to make other people happy. I need to live a life that is consistent with my personal beliefs for my own well-being---even if others do not approve of my decisions. I tried to live the Mormon life for many years despite not believing in it. That was a mistake. It caused me to experience intense depression and severe anxiety to pretend to be a believer when I was not. I needed to leave Mormonism in order to have peace of mind. It was not easy to leave, but I feel a lot happier outside of the church than I did when I was inside.
I will continue to be your friend if you would like me to. I will continue to treat you with respect and warmth. I would hope that you would extend me the same courtesy. Thank you.
Generally in these kinds of conversations with friends, my goal is to be clear and honest about my reasons for leaving without going into a lot of detail (because I don't want to cause the other person to get too defensive). Since these kinds of conversations can be a game-changer for some relationships, I usually try to make it clear how I will continue to act in the relationship in the future, i.e. with continued respect and kindness. I tell them that I would hope to be treated the same way in return. If necessary, I sometimes add a "boundary" (like a clear behavior request).
Family members are a little different. In the first place, it's fairly rare for a family member to want to discuss my relationship with the church directly and openly. But when they do approach me, I tend to be less direct in those kinds of conversations. For example, when my sibling who is on a mission bears testimony to me in an email, I usually don't respond to it directly and I tend to change the subject. Since sibling relationships have much higher stakes, I would prefer to have those conversations in private and face-to-face where I can communicate with love and respect. In other words, my goal with those kinds of conversations tends to be to forestall discussion until the conditions for a conversation are more ideal. Honestly, I'm also fine with avoiding the topic and just letting it be the proverbial elephant in the room.
I'm not sure about the ideal way to handle these conversations. I don't really think there is an ideal way when both parties disagree so fundamentally about the key issues. Especially when it would take several days/months/years of library research to get to the point where a meaningful exchange of ideas about the church could occur.
Most people haven't done the scholarly groundwork to really have an in-depth conversation with me about polyandry, the theodicy, Biblical history/archaeology, church history, Book of Mormon/Doctrine and Covenants/Pearl of Great Price textual criticism, freemasonry, etc. etc. etc. I'm not saying that to be elitist; I'm just saying that until people have done their homework to make sure we're all having a conversation on equal terms with equal access to the same sources of knowledge, then we're not very likely to have a productive conversation. So, usually my goal is to try to communicate that I still love and respect the people in my life, while at the same time remaining true to my own sense of self. If someone does decide to take me up on the library challenge, then we can have a dialogue. But I'm also content to let the status quo remain.