Long ago, our Father in Heaven gave us the opportunity to choose for ourselves. This gift of free agency for His children came at no small cost. A third of the hosts of heaven followed Satan, and were cast out of heaven before this world was. But the other two thirds chose to follow God's plan, to follow the Savior...and because of that choice, were able to come to this earth, receive a body, and be tested in a mortal experience on earth. You chose to follow the Savior. That is why you are here on earth today.
We learn and grow from choices we make here. The Mormon.org website explains this clearly: "God influences you to do good and to follow Him, while Satan tempts you to ignore God and commit sin. When you commit sin, you separate yourself to some degree from God. " He knew that we would make mistakes and make wrong choices at times. Fundamental to His plan, He provided a Savior who would atone for our shortcomings, and make it possible for each of us to repent throughout our lives. Following this plan would enable us to return to live with our Father in Heaven again.
The choices you make determine whether you will be happy, as God wants you to be, or miserable, as Satan wants you to be. President Henry B. Eyring said of this,
The spirit who led the rebellion in the world before still opposes the plan and wants you to be miserable. He wants you never to find your way home again. That enemy of your soul knows you and your goodness. He knows that if he can turn you away from walking in the light, he can both capture you and stop you from helping others along the journey.
He knows how good you are and your power to teach and influence hundreds of Heavenly Father’s children in this life—and thousands over the generations that will follow your path. If he can get you to wander away from the light on your journey, he can do harm and bring misery to many.
So the battle rages over the souls of men. This truth is taught to young and old, in local priesthood quorums, seminary and Sunday school classes, and youth meetings: that there are many influences in this world to do right, and influences to do wrong. Fortunately, when you were born, you were given another gift from God that goes hand in hand with the gift of agency: the Light of Christ. It is manifest in your conscience, that inner sense of what is right or wrong, what is true or false.
Know this, that great blessings are available to you now and in the eternities when you make correct choices, to pursue what you know is true and good, deep inside. To gain the peace and happiness you seek, continue to follow the Savior as you did before. It is a promised blessing.