Forget It, I'm right there with you with the cycles thing. I'm feeling a bit "up" right now, so the looming periods of despair don't bother me as much, but I know they're just around the corner.
It helps me not to think of my life as happy or unhappy at any particular moment. I've had days when I'm optimistic and, dare I say, bubbly, in the morning, but extremely depressed and suicidal by late afternoon. And you're right: the fall is a lot harder when I've been higher than usual. But in most cases, my days are a more complex mixture of the two, so this fall isn't so shocking.
Right now, my life is just...okay, which is fine with me. There are some things I'd really like to change, but I can't say I'm miserable either. I'm old enough now to know that I'll never be "happy" the way some people are. But as long as I'm okay, I don't feel bad about not being happy. In all honesty, happy people usually annoy me, anyway. The world is a cruel, unjust place controlled by like-minded people. The only way to be happy all the time is to be severed from reality.