I'm upset with my best friend while she's going through some stuff
Why are you upset with your friend?
For talking to some fuck face on the internet.
You should take the time to respect what your friend is going through. Maybe you can wait until they don't have so much on their plate, and talk to them then.
They keep avoiding me and I don't get why. I'm done with them, they don't know how to treat a friend.
Let your friend know that you care for them and are there for them, and then give them some space. They might not be avoiding you, but rather just spending time alone.