I have learned that I'm not much good to others unless I've got mental energy. I do think that we are capable of pushing ourselves much further than we usually do (meaning we probably tend to retreat too soon, most of the time), but the point still stands. We must retreat (it's who we are) and then come back strong. Instead of being "effective" in society from 6am - 10pm every single day (like most ESTJ's with a work first mentality), I like to say that I'm effective in "fits and starts". For 1-day bursts. Or a 2 hour burst in the morning and then a 2 hour burst again later that night. I'm really in tune with my energy level and I'm learning to play off of that energy. When my mental energy is high and I have a lot of clarity, it's important to put that time to good use. When my vision is cloudy and I'm mentally overwhelmed, then it's time to retreat. I can still use this down time to work towards doing the things I want to do. I can plan things out, brainstorm, create (on paper), etc. Then, when the energy is back, I can go out and put those things into the real world.
In regards to the tendency to withdraw altogether (and be less committed to life in general), it's important to decide what exactly it is we want in life. Not very long ago, my signature had the following quote by Ben Stein:
"The indispensable first step to getting the things you want out of life is this: Decide what you want."
It sounds so simple. But, once this step is hashed out and decided upon, then the vast number of ways that it can be accomplished opens up to us. If we never take this first step in deciding what it is we want, then how can we ever move towards accomplishing it? Or how can we use our high energy times and our low energy times toward that end, if we don't even know what that end is?
I loved your post. It describes my experience as an sx/sp very well. You described it like a wave. A high time and then a "crash" so to speak. I think it behooves us to accept that fact. It is a wave. And we should learn to use those waves (our up and down energy) to our advantage. Think about if you are in the ocean and waves are coming at you (if you've ever done that). If you try to fight the waves, they get the best of you - every time. If you try to go against them, you lose. It's futile. But, once the light bulb goes on and you realize that you can ride the waves - the power provided by those short bursts can be used to propel you forward - the whole game changes. It becomes fun, it becomes enjoyable, and you learn to make progress. It's still a challenge and you'll still take some hits, but you get better with experience.
Where this all ties in together, is that you say we "WANT". It's a powerful want that we have. So, once we "decide what we want" (as Ben Stein says), then it's just a matter of using the waves of our changing energies to move forward toward that want.