Dear Lester:
I’d tell your boss and apologize for not telling him sooner. I would tell the boss and apologize in a self-respecting way, in a confident way, not as a man tortured by guilt and anxious. Telling and apologizing in a simple, straightforward way, in a confident, sincere, pleasant way will give the boss (if he/she is wise enough) the information that you are reliable, trustworthy.
Don’t make a mistake grow into a huge mistake. Catch it now and repair.
Of course, you may get fired once you tell, that is the risk. This is why it is your choice to make.
anita