I had a posterior fossa Arachnoid Cyst, and I eventually had fenestration surgery 12 years ago. So, I know about the pain and other problems they can cause! Many Neurosurgeons/Neurologists say they have been there all your life, and that is mostly true, but they can grow over the years and eventually cause many problems, like you are having now.
You need to try and find a Neurosurgeon who will take you and your symptoms seriously, and possibly operate. But I know that is not always easy, and I can't really help you with names of good neurosurgeons, because I live in Australia.
Best wishes,
~Jaime~