I'm so lucky to have such a great friend like you... who'd forgive me if I forgot his(/her) birthday. Happy belated birthday!
Wishing you a belated happy, happy, happy birthday. Sorry, I missed your special day.
Yes, it's true that I completely forgot your birthday but I think I should get some points for remembering I forgot it. Hope you had an incredibly wonderful special day!
I'm sorry I missed your birthday, but you've had so many that I've lost count. Wishing you a belated "happy birthday!"