You know, with the seat so upright, you may in fact be putting all your backward force near the shoulders, causing flex right at the point where the seat broke. A couple of questions...

1) Is the seat distance to the pedals correct? Within no more than about 1-1/2", the bottom of the seat back should be the same distance to the most forward part of the pedal stroke as your X-seam. X-seam is the distance from the wall to the balls of your feet when you sit flat against a wall.

2) Are you pulling and pushing on the handlebars during the ride? Doing so varies the forces you place on your shoulders and thus the top of the seat back. This may also fatigue the handlebars so much they also break.

:)ensen.