It is my birthday today and I feel like a pretty selfish and horrible person.
What makes you think that you are a horrible person?
I ignored my mother's happy birthday song.
Birthdays can get busy but we should remember to think about others and appreciate their gifts. It is okay to feel bad that you forgot to thank somebody when you did.
I don't need your pity. I just celebrate my birthday on my own and will feel sorry for myself.
I feel like you would prefer to celebrate your birthday with your family members. Do not beat yourself up about one little mistake on your birthday. I am sure your mother will understand if you apologize.