In a correct sliding stop the horse slides with his rear feet with his legs underneath him and keeps moving his front feet...it takes alot of steps to perfect a sliding stop. Start at a trot, say whoa and sit deep and take your legs off while supporting his face. Then back up a few strides.....again and again, until both you and your horse get the feel of it. Also there are many exercises that can help to encourage your horse to get deeper in the stop. There are many good books and videos that can help. The best use of your time and money is a few lessons from a trainer until you both get the hang of it. Also is your horse shod with sliding plates? Without them even the best stopper will hurt from the impact,, and do the skipping you describe. Or as mentioned before sore hocks, back etc.