What you're doing wrong - Just a guess, but if you shoot good for awhile and then wind up lobbing one over the top, it sounds like you're shooting too much, getting tired and letting your concentration slip away.

About the back stop... It's tough on arrows, but it sounds like the car trailer is doing a darn good job. I've made some good backstops out of carpet remnants. You can either cut the carpet up into strips 2' wide and 4' long, then compress them into a bale with banding straps or make a layered, free hanging backstop.