I go through those moments too. I hit the lowest of lows and start to think I can't get myself out and suddenly I have this rush of energy and new outlook that lifts me up. The good days are what keeps me going and just like you, I realize they won't last and I will end up feeling depressed or over emotional about something. The trick is to try and avoid the triggers and try to implement the things that make you feel happy, more into your life. It doesn't completely get rid of the bad moments, but it at least lessens them.