I missed my good friend's birthday party last month because I was too busy and forgot.
That's such a shame! Were you honest with your friend, and did they forgive you? 
I was honest, they were still upset which makes me feel bad. I tried to make it up to her.
If she is that good a friend, I'm sure she'll forgive you soon enough. 
I hope that she does, it made me feel awful. But here's to good thinking and positive vibes, she will come around!
sending my positive vibes to you too! Fingers crossed! 