This verse is speaking of the coming of our Lord, Jesus, but I believe it can be applied to our lives. Jesus can make the crooked places of our lives straight, and He can make the rough parts of our lives smooth. He took on Himself all of our sins, removing the stain of sin from us and making us "white as snow." From that point on, the past is gone, that's right, GONE forever. Our sins, our crooked and rough ways and paths, are never to be remembered by God as if they had never happened. It is important for us to do the same with those difficult times of our lives. Once we have accepted Jesus, we become new creations as we are told in 2 Corinthians 5:17, "Therefore if any man be in Christ, he is a new creature; old things are passed away; behold, all things are become new" (NKJV). Those old mistakes, those old sins, those old habits are gone, passed away, and now we have become new. Although we sometimes find it hard to forget the past, God already has, and He expects us to do the same.
"Every valley shall be filled and every mountain and hill shall be brought low, and the crooked shall be made straight, and the rough ways shall be made smooth" (KJV).