Simply replace the unfilled areas such as "" and "[date]" with your information.
I just wanted to take a few minutes to tell you how much I really appreciate everything you've ever done for me. I know I wasn't always the easiest kid to deal with, and I'm sure I tried your patience quite a few times. But I know that you always meant the best for me, and I am proud to call you my [Mom/Dad].
I hope that one day, I can be the type of parent you are to me to my own child!
Happy Birthday, [Mom/Dad]. I hope you have a really great day, and a really super year. You deserve it for all the work it took to raise a kid like me.