Luke it sounds like you are really thowing it to hard or your not taking your head out of the rotation and stopping it. I would say that it is for sure one of those. For me, doing TS Back Rolls or TS r2rs I think about not hucking the rotation, but riding up the wake and pushing my board up and over my head. A lot of people just try and rage it like a HS Backroll, that will make you skip out or land on your butt. Try and take the trick high and slow. This will get over the second wake so you dont case and allow the trick to stay incontrol.
Hope some of that helps, but Like Nick said, really this is one your going to want to do, over and over and over again. I would also suggest trying to grab while learning to trick. This will help you avoid the dreaded straight leg backroll that so many people do, that leads to bad habits that make it crazy hard to grab.