The principle of agency is taught in great detail in the Book of Mormon. Alma teaches his son Corianton personal responsibility for our choices in this way:
3 And it is requisite with the justice of God that men should be judged according to their works; and if their works were good in this life, and the desires of their hearts were good, that they should also, at the last day, be restored unto that which is good.
4 And if their works are evil they shall be restored unto them for evil. Therefore, all things shall be restored to their proper order, every thing to its natural frame—mortality raised to immortality, corruption to incorruption—raised to endless happiness to inherit the kingdom of God, or to endless misery to inherit the kingdom of the devil, the one on one hand, the other on the other—
5 The one raised to happiness according to his desires of happiness, or good according to his desires of good; and the other to evil according to his desires of evil; for as he has desired to do evil all the day long even so shall he have his reward of evil when the night cometh.
What are the desires of my heart? What do I want? No one consciously wants misery, but we often choose evil over good. The more difficult decisions in life are sometimes choosing good over good. When the Lord offers us so much in life, we sometimes are confronted with multiple good things that we can do. If we listen to the voice of the Spirit, we can not only learn to consistently choose good over evil, we can become sensitive enough to be able to choose between good, better, and best.