I know it may seem easier to write a letter, but it will probably work out better if you tell your father in person.
Tell him you're still saddened by your mother's death, as you're sure he is. It is a huge and painful change in both of your lives. Tell him that the way you are feeling is making it hard for you to concentrate on piano lessons. Ask if you can put them off for, say, a year.
If you both talk to your piano teacher, you can probably have the payment applied to next year with little trouble.