Just focus on relaxing. Your heels down was great, if you were 2-Pointing it, try and work on getting that steadier. You can focus on doing this easier if you get someone to lunge out your horse, so you don't need to worry about steering and directing your horse. If you were sitting it, keep your legs long and relaxed, and your hands steady. The horse did awesome toughing it out, I hope he got a treat. He may have stumbled because you jerked him. You looked great for your first try! Keep going, you'll get it!