I quit teaching very recently (I have a Masters in English) to pursue programming as well. In all careers you make a lot of mistakes, but with teaching, you only get another crack at the problem once a year. If you create a terrible lesson, you agonize about it, and you might not get it right the second time either.

Add to that the fact that schools are always really slow to change, requiring you to fight for anything remotely progressive (the school I was teaching in graded multiple choice entrance exams by hand); it's really easy to get sucked into the traditional educational model even if you don't believe in it; and teaching doesn't allow for the creativity I want in my life.

I suddenly realized that I wanted to be creating not evaluating — to be always learning and building on what I know. Programming does seem to offer that.