For a lenient bit for your horse, try the Duo 16mm White Plastic Eggbutt Bit from Sprenger.
Sprenger Duo bits are very gentle on the horses mouth. The mouthpiece is made of soft, flexible plastic, which is food-safe, solvent-free and does not contain any plasticizers. The mouthpiece is reinforced with a steel cable to make it stable and secure. No returns or exchanges are possible if there are chew marks on the mouthpiece.