You would not put up a huge jump for an inexperienced youngster, so why ask an inexperienced youngster to do advanced work that his body is not ready for?
This is particularly true for maintaining impulsion in the trot. It takes months for the horse to build the muscles and stamina to maintain a good level of impulsion. Be patient.
If you do not do sit-ups regularly, get down on the floor and see how many you can do today. Do the same tomorrow and the day after, they’ll be getting harder! Think of your horses development in the same light, it will get easier but it will take months for you to build up to 50 / 100 per day.
Think of each sit-up as the equivalent to your horses trot steps using impulsion. Start with 5-10 steps and build up very slowly until your horse is fit enough to easily execute this very, very demanding work.
Listen to your horse, if he cannot maintain the work, he could be tired, or have aching muscles. Let him rest and try again next time.