I take no drugs, unless being addicted to the gym is a drug. Exercise is relaxing, meditative and allows me so sort through my thoughts.
If you have been operating on 5 to 6 hours of sleep over the last 5 years, hopefully you have been eating and living properly. Unless you made some radical change recently you should be cool.
Overreacting might be the case here. Think positively, it should all work out fine. You are hard working, ethical, and you hold yourself to higher standards than others, you will not lose.
In the future, ask yourself better questions to keep from fretting. Keep in mind not to become upset over people or things out of your control or sphere of influence.
In regards to this situation think of ways you can improve your transactions with the customer and keep your stress levels down, make a checklist of things you can do differently to avoid this in the future.