Once you've achieved something you never thought possible how do you climb down from that hieght?

It is a buzz but like all excitement it diminishes over time and you strive for another achievement.  But inside you have the glow of achievement.  Pride if you will. After a while it gives sufficient measure of self confidence that no one can successfully belittle your self worth.

You don't unless you want too or you seek out another mountain to climb as for some it's the journey...

Easy. I remember how to spell "height", and then I just come back down.

Plenty of other achievements that need to be accomplished -  so just move on.