This is really awesome. But please -- save this as a note for some other time for your mother. As a dad, it'd be heartbreaking to get a "it's time to let go" note on Father's day. It would crush me. Even the most amicable divorce is tough.

Give your mom some flowers and, heck, draw something for her. Or use your talent to express your appreciation for her. But don't give her a goodbye letter on Mother's day.