I also experience these lows quite often, about once a month. They usually don't last for more than 3 days, and all my energy and passion come back. It's still tough to deal with, especially when you suddenly feel like there's no purpose to the things that just yesterday gave so much meaning and excitement to your life. I learn to cope with them by training my mind to take a moving average of my feeling. This way I'm constantly reminded of my overall long-term sentiment.