I've been thinking a lot about living one day at a time. It is so easy to dwell on the past or obsess about the future, neither of which is healthy.
When we dwell on the past, we harm ourselves because it can destroy our hope. Elder Neal A. Maxwell encouraged us to hold on to hope by "not looking back, and refusing to let yesterday hold tomorrow hostage."
Think about Lot's wife in the Bible - she was told not to look back when they were fleeing from Sodom. She did look back and was turned into a pillar of salt. In furthering this analogy, salt is not a living substance. It cannot change or grow. I don't want to be like Lot's wife. If I look at the past and dwell on regrets, I will be like the salt, unchangeable, stagnant and not thriving, albeit spiritually in my case.
A friend of mine gave me another example of this concept. She said, "You wouldn't replay a bad movie 200 times would you?" So why do we replay our own (or others) negative experiences over and over? Whether the bad experiences come from our sins/mistakes, someone else's sins/mistakes, injustices, tragedies, illnesses, rejections, etc., they are in the past. Just throw out that bad mental DVD and start making a better movie in your mind so to speak.
Bottom line, we need to MOVE ON and realize that our hope isn't based on how bad things are or were, but how good things can become with God's help. This relates to one of my favorite sayings from my teen years, "No matter what your past has been, your future is spotless."
Discouragement comes from missed expectations. That is why I think it is important to fill our hearts with gratitude for experiences we've had in the past, gratitude for what we have in our lives right now, and hope for a future full of growth, happiness and learning from trials. Look for those tender mercies that come from the Lord in our lives every single day.
As far as obsessing about the future, it is good to have hope for a brighter tomorrow and for happiness to come. But it is unhealthy to allow doubt and fear about the future to control our hearts. Surrendering our timetable to the Lord and trusting that He will give us the experiences we need (or even something better than we could imagine) is the philosophy I choose to have. "In times of distress, we can hold tightly to the hope that things will work together for [our] good."