I will ignore and cut off my best friend when she needs me most
Why do such thing during your friend's most down moment?
I'm going to do it because we haven't been friends for a long time and I can't be bothered at this point in time.
Friends do grow apart, but it is not good to not be there for a friend when they really need you. It could give you the opportunity to grow closer together again.
Why do I have to feel responsible for her feelings?
That's what friends are for. They care for each other and exist for support.