I feel bad for rarely seeing my extended family but every time I do it's just terrible, and I go insane.
That's unfortunate. What happens to make you feel terrible when you do see your extended family?
They don't seem to understand that even though we're blood related we're not in fact close.
It is understandable to not want to see others who are not close to you and makes you feel terrible about it. But you should always make time for your extended family even if you are not close to them.
I usually just ignore them when I see them.
You shouldn't be so rude to your family. Extended families can be extremely fun when they get together. Try engaging with them and you'll probably have fun with them.