It happens man, don't sweat it.
I was about to say something but another member beat me to it. It is pretty clear from your pictures that the pep pieces bubble and fall into themselves here and there, and thats exactly what happens with regular paper. The reason cardstock is used is that it is more rigid, and it soaks up the resin much better without warping (if braced correctly, that is).
Just be very careful on your folds when you get some cardstock. The work you do on your pep will follow you through the rest of your build, so work to make those folds clean and crisp!
Good luck and stay patient, its a loooong project, and it is so worth it by the end haha!!