The two biggest lies we can tell ourselves are "Things will always be awesome forever" and "Things will always be awful forever". On the high of a great new thing or the low of depression it can be so easy to view the present feeling as the true nature of the world, rather than as the current weather.
By forgetting that these peaks and troughs are just that, exceptional highs and lows from our normal levels, we cut ourselves off from treasuring the best moments as truly precious and from taking hope in the worst moments that this too shall pass. Delusion and despair come when you lose track of the sweep of your life and of your capabilities to get through the full spectrum of your experiences.
As C.S. Lewis put it in The Screwtape Letters, "periods of emotional and bodily richness and liveliness will alternate with periods of numbness and poverty". Embrace this; there are valuable lessons to be learned in both states.
Extreme joy is a wonderful thing and well worth attempting to bring into your life more often, but its intensity can also leave you appreciating the quiet pleasures of your normal, middle-of-the-road days. Being extremely up also has a risk of shielding you from a realistic assessment of some problems and how you might perhaps be contributing to them. Deep gloom walls you off from daily satisfactions and from believing in your own worth and the skills you have which can elevate you out of the doldrums or improve your day-to-day life. But depression also is when your good habits have the greatest chance to show how fully you've brought them into your nature; when ya don't wanna do what's good for you, but you go ahead and do it anyway, that is when you show your greatest strength.
When you return to your default footing is when you have the greatest perspective and the greatest opportunity to contrast your perception while at the extremes with your present, more clear-headed view. Ordinary days offer you the most opportunity to identify the changes you want to make in your life and give you the most balanced set of your strengths to bring those changes into reality.
Return to the big picture often—journals or mood logs help build that bridge between the highs and lows—and keep reaffirming your happy dreams and your ability to make them come true!