I never thought I would let something as important to me as your birthday slip by. Unlike you I must be getting old and forgetful. I hope that you had a pleasant day filled with enjoyment and delight.
My only consolidation for this is that I will have many more opportunities in the future to help you celebrate your special days.
I would like to take you to dinner when you have a free night to make up for my mistake. Call me with a time and where you would like to go.