Hi Surfer,
I, too, have had an arachnoid cyst show up on an MRI last month while my Consultant was looking for causes of another visual problem. This explains the emotional rollercoaster I have suffered for years, but unlike yours, I have become as unstable as a 13 yo girl - embarrassing for a 53 yo ex biker!.
Brain surgery of any kind is terrifying, but medical techniques today make them as safe as possible… and everyone on here will be rooting for you. Has your Consultant discussed the possibility of the cyst recurring after surgery?
Finally, don't underestimate the power of the love you share with your dog. I lost my Bullmastiff, Bobby, to cancer several years ago but still get my endorphine fix from Jasper, my daughter's Collie, every month.