I believe it is the balance of nature to bring you to the lows so that you can appreciate your accomplishments. Just envision yourself sitting at the bottom of a hill mesmerized with the beauty there must be at the peak. The thought of the breathtaking views at the summit can be almost dreamy and inspiration enough to keep going.
I can proudly say that I have been through a few periods of 2 pi of these curves (geeks can chuckle :P). My experience says that to make the best of the time when you are moving along the negative Y-axis is to look back and enjoy what you missed when you were busy winning the game of life. It is very important for you to savour the blessed days and stay motivated to go uphill again. Take this opportunity to thank those who have been by your side all along.
Back in school we used to sing a hymn in the morning prayers which now makes so much more sense.
"Count your blessings name them one by one....and it will surprise you what the Lord hath done"