Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [your preferred term for your Mother],
On this Mother's Day, I not only wanted to wish you a Happy Mother's Day, but also wanted to wish you many other happy days as well. I don't know anyone that deserves a good solid bank of happy days more than you.
You have always been there for me, to support me when I needed a hand, to push me on when I needed to let go. You have let me cry on your shoulder, let me yell when I was angry, and let me talk for hours when I just needed to get it all out.
I love you, and appreciate everything you have ever done for me. A Happy Mother's Day is the very, very least I could begin to wish for you in return.