I completely forgot my good friend's birthday yesterday?

[Living♥]

Yesterday one of my best friends turned 17 years old. I know she definately considers me to be a close friend of hers. We tell eachother everything, and go on big road/weekend trips in other towns and stay in hotels and all together, etc. She completely remembered my birthday in September, and surprised me with a card and gift and everything. I didn't even bother TEXTING her! Even my mother remembered to wish her happy birthday on facebook. I feel so awful, especially since I hung out with her Wednesday night, the night before her birthday. Tomorrow I am going out and getting her a birthday card and present... Ideas on that? And ideas on how to make it up to her/let her understand I didn't mean to forget and care about her as a friend?