Four years ago I met an individual who has played a large role in my development and who I'm lucky to call a close friend. He was presenting at a convention I attended and this is a lesson he shared that has helped me through tough times.
Most people have high expectations of happiness. They believe their happiness should always be increasing. Graphed, their expectation of happiness over time looks like this:

What these people fail to realize is life is a series of peaks and valleys. No matter how hard they try, they cannot always be on an upswing. Because they don't accept this fact, they are devastated every time they hit a valley.
Instead of attempting to always be on an upswing, your goal should be to trend upwards. Work to extend the upswings higher and longer and attempt to shorten the downswings. Similar to the graph below:

Once you accept that downswings occur, they can become a source of growth instead of a place for despair.
So, if you find yourself in upswing enjoy it with all your heart. And if you find yourself in downswing, utilize it to catapult you even higher.
Strike the Match. Take Action, Be Extraordinary.