While looking into what is already available, I found this breathing exercise that is meant to help you focus before you set off for a ride.
Try it here and see if you feel up for it.
If you have no previous experience with breathing exercises, I would recommend NOT to try this as I find it quite difficult for a start. Instead just sit down and try to focus on your breath. Follow it as it enters the nostrils and travels through your body and watch it as it goes out. The more you manage to calm your breathing, the more relaxed will be your mind. Try not to think of anything. If thoughts enter your mind, acknowledge them and let them go. Try counting as you breath to help you focus on your breath.