How To Withdraw Pristiq?

Hello. 6 years ago at the age of 55 I had an subarachnoid hemorrhage at the top of my spinal cord, resulting in a number of surgeries to repair, and installation of a "brain shunt". I had much anxiety & depression, and ended up on 100mg Pristiq, then 5 years ago went up to 150mg / day. The hard question is how much my condition is due to the Aquired Brain Damage, and how much is due to the Pristiq. My condition since the ABD is short-term memory loss, often confusion, cannot prioritize, am overwhelmed with small tasks, and eg - just the concentration necessary to write this, is wearing me out to the point of me needing to sleep to recover, or I won't be able to talk, or walk. I can concentrate very little, and my wife often needs to care for me, like a child. I am unable to work, and just want to sleep.
I would like to come off Pristiq to be sure it ins' making things worse.
My doctor is unsure how to reduce my dose, how do you think this might be possible?
Lindsay Trigger (Australia)