I'm not going to my best girl friend's birthday party
I'm really concerned. Why would you not go to her birthday party?
She's my best friend. I'm not going to hurt her for some party
You should be there for your friend's birthday. talk to them and if you are not able to go to the party, find another way to support them.
At least, I know I don't have to worry about her moving on with her life.
It's not good for her to move on if you ended things on a bad note or to avoid her in your life. If there's a party that is being hosted by your friend, then you should go so that they know that you support her.