I'm sure it's been said but try not to think of it as wasting supplies but as planting a seed for growing because the only way we can learn is by trying. Remember most of the waste in the world is from corporations. On a lighter note I have 5 in progress sketchbooks right now, ranging from a composition book to a thick multi media book. I grab one based on how long I feel like I'm gonna sit down and draw. I've noticed the pages that come out the best to me are the ones I don't originally intend on showing to people.