I know exactly what you mean. INFx types are the most depression prone as a starter, but weirdly when I'm down I feel more alive than the mid point where I am most of time time. I write a lot of my best songs and words during these periods so I almost look forwards to them. Often just as I'm in a really happy mood I think to myself 'this is just a stopping point to me being unhappy again' and I wish I didn't but I just do.
It's a strange sensations, like standing above a raging torrent with a blindfold. You can hear the roar and feel the spray on your cheeks but you're not actually in the river, just dipping a toe in now and again and the icy water chills you to the core.