I don't think it's being dumb at all. It may be that it's a mild attention deficit disorder, but that can be treated. And I would say that probably a psychiatrist could recognize it immediately and medicate you properly so that you are able to concentrate just as you wish.

So many times these things are reactions from diets that need to be corrected.
If I were you, I would sit down with a psychiatrist, discuss your foods in your diet and ask about attention deficit disorder and whether he thinks you have a touch of that.

You're judging yourself too strongly, which indicates to me that you have a very strong conscience and are taking too much blame for something that may just be a chemical imbalance somewhere. Your employer will be very understanding if he is a mature person and will help you along if you are serious about wanting to correct the problem and working with meds and getting help from a specialist to get you past the hump.

Take care and good wishes ahead.