I agree with hvert that what you feeling is normal and typical for a new job, especially when you have been out of the 'paid' workforce for a long time. Putting in some extra time on your days off to self-orient is an excellent idea.

If you are particularly squeamish about teaching the course you have only just taken yourself, you could ask your boss to let you see how the learning goes for you before you commit to teaching it to others.

I imagine you are doing very well in your position but your brain is telling you otherwise. It will just take time to settle in until you become convinced that you are competent. Take it easy is the other aspects of your life as you go through this process ... soak in a warm bath in the evening, read a magazine, etc.