When driving someplace, get behind the slowest person on the road. Do not pass them, do not ride on their bumper. Just follow them, match their speed, and watch your attitude closely.
If you're a speedy person like me, you're usually annoyed by slow drivers, and stress levels rise if you're stuck behind one. Even if you're not going anyplace in particular, or even if you aren't under any time constraints, you are conditioned to want to drive at a certain speed, and when that is impeded, you're thrown off guard. I've ridden with many people like this and have seen it in myself.
Use the controlled situation of a voluntary slow-down to learn how to relax yourself when stress appears. Relax, remind yourself that you'll reach your destination, and nobody will notice that you're late. If they do, then you were really late to begin with, and being behind a slowpoke didn't make a difference. Figure out how to induce calm in yourself, and then you will develop a talent which will apply many other places in your life.