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"Light dispels darkness. When light is present, darkness is vanquished and must depart. More importantly, darkness cannot conquer light unless the light is diminished or departs. When the spiritual light of the Holy Ghost is present, the darkness of Satan departs." Out of Darkness into His Marvelous Light (emphasis added)
My conclusion to the questions stated above replacing ups with light and downs with darkness is that as we do the things we know we are supposed to do light or happiness fills us. We allow Satan or darkness to overtake us by sinning, and our light diminishes and will eventually depart. One important thing Satan doesn't want us to realize is that we still have the ability to light that match or flip the switch; causing light to overtake darkness once again. Because of Jesus Christ and the sacrifice he has preformed the option will always be there. The choice is up to us. This doesn't mean it will be easy or even that our light will be as bright as it once was. But as we try our best and apply the atonement of Jesus Christ through repentance all that seems lost will be found, all that seems impossible will become possible, and every wrong will one day be made right. I know we can only see a portion of the big picture of life right now, but as we remain faithful more up times will come and all things will work out in the end.