Why does my horse stop when I say whoa lunging but when I say whoa riding he throws his head & flips

Ok, I will try to answer this in a simple understandable fashion. It is a great thing to work your horse from the ground and lunge to build communication and trust between you and your horse. However the horse only learns that at some point in time when he or she has done everything you have asked there is a reward and released of his duty, his freedom to end session. Lunging does not actually get that energy out it builds the horse up to expect something is expected of him. To burn energy out, mount in a relaxed fashion and work together at a relaxed pace to get that energy out. It is totally different to build trust and understanding from the back. Here is where you should spend more time bonding with your horse than lunging, he will learn that working together gives him that same release of freedom when finished on a good note. Try to make it a point to spend one hour every morning or evening mounting your horse and simply working on communication of stop and go, be relaxed & light in hand so as to communicate that when you drop your seat, heals & hands you are asking the horse to stop then reward with praise. Try using a Bosal instead of a bit to start so you working on communication with the whole head, not the mouth to avoid pulling on the horses mouth teaching resistence which enforces the throwing up of the head and resistance to the stop. Your body language should be all that is needed to communicate the whoa standard for stop. To communicate with your horse it takes lots of repetition and patience from the back. When you ask the horse to whoa by dropping your heels and relaxing your lower spine into the seat, relax your hands, if he does not stop with you... lift up on the right hand lightly at the same time to get his attention, if he does not whoa lift again and apply a little more pressure as if to turn his head slightly to right, the instant a slight stop occurs relax and praise until he gets it. Repeat repeat repeat and end on a good note to let horse absorb what just took place, horse says hmmm "when I stop this person releases the pressure, maybe it is good to whoa". Remember to be relaxed and communicate mentally and look to the direction you intend...stop means mentally all your motions have come to relax and stop, drop, lower your nose slightly & relax with your horse. Good luck and keep at it.