How do I tell my teacher without being made fun of that I can't tell my d's and b's or my n's and m's apart?

You won't be made fun of by your teacher. She probably already knows your problem. Meet her in private, take a parent and discuss how you can get assistance.

What grade are you in?

This should be part of your medical and school record. Do your parents know? Your teachers should have been notified your first week.

Was. Saw. Which one cuts wood?

I'm surprised that your teacher has not recognized the problem yet on their own. I understand you feel embarrassed, but there is nothing to be ashamed of. The teacher will understand, and no one will make fun of you. Stay after class one day and talk to them, they will guide you through this.