Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of spouse, or endearment],
I hope that you have a most perfect day on your birthday, and send to you a plethora of wonderful wishes for the most wonderful person in my life.
I love you, and want to spend many more years with you. Thank you for everything you do for me, and for always being there when I need someone to talk to or just to sit in quiet silence with.
You are my sunshine and my shade tree - you warm me when I am cold, and cool me down when I get too hot.
Love, forever and always, to the best birthday [boy/girl] in the world.