What would you do when blamed for things that are not your fault at work?

I work at a Chamber of Commerce. There are only two people who work here (my boss and I) and there is a Board of Directors. I take care of all the marketing, including post cards, emails, website and brochures. I made a mistake on a postcard I created for a member of our Board of Directors. I wrote the time for a seminar as PM instead of AM. This was a major mistake because the postcards went out in the mail. I could have sworn that my boss told me to write this, but could didn't have proof so took the blame. Then I found a note in my recycling bin explaining what to write on the postcard and it said PM (with my bosses handwriting). I have to deal with this person on our BOD a great deal. She talks to me like I am a complete moron. This is compounded by the fact that I made an spelling error once before on a postcard (last year). Now I am making a different postcard for her and she claims she sent me all the information for it. I know she did not because I save all my emails and don't have it. She thinks I did get it and this is again my error (as if I am prone to mistakes). Should I just let it go? At this point is seems a little silly to throw my boss under the bus, but dealing with a person who talks to me like I am dumb is annoying. I am planning on getting a new job soon anyways.