Thanks for writing to JA.
Your question is intriguing, but I feel the need to clear up several misconceptions.
#1. You should understand that Multiple Personality Disorder (MPD), now called Dissociative Identity Disorder (DID), is a very controversial diagnosis. While there are many in the field who believe in its existence, there are many who do not. Further, there is research that has demonstrated that MPD/DID can be faked and that therapist behavior can reinforce and, by theoretical extension, "create" MPD/DID states in normal individuals. This suggests, again by extension, that if MPD/DID states can be created among normals, that it can very easily be "created" among fragile clients with significantly abusive backgrounds. This theory is called iatrogenic meaning is derives from the healer.
#2. Please understand that I am not questioning your diagnosis. It is inappropriate and unethical to diagnose over the internet (and across the Pacific Ocean).
#3. There is no evidence whatsoever that individuals with MPD use "more parts of their brain" than normals. The evidence behind the urban myth that we only use 10% of our brain is also hokum. While, at any one second, we may use only 10% of our brain - this amazing organ is constantly in activity. A split second snapshot of a racehorse would make it appear that it only uses its legs to run... but looking at the racehorse over time, we understand that the entire horse is engaged when running down the track. The same is true with the brain.
#4. To my knowledge, there is no known mental, emotional, or psychological cure for a brain tumor. This falls under the purview of a neurologist or oncologist (depending upon the nature of the tumor).
In short, I hope you will work collaboratively with the team who is treating you for your brain tumor. I don't believe that there is anything you can do, personally, to affect the growth of the tumor - other than to live a healthy life, support yourself with people who love you, and follow the recommendations of your physicians and nurses.
I wish you the best of luck.