Stacy,
First of all you are not alone, which is comforting, yet knowing others have suffered so much with such a cruel disease as Cancer is also sad. I am an only child and 3 months ago lost my Mom "Best Friend" to Pancreatic Cancer. I also am a Psychotherapist and I am familar with and have counseled others with the Grief Process. However, you never understand until it affects you personally. Whether it is the Shock of someone being diagnosed when they were Healthy just Days before, the speed and deterioration of this disease and also reality of missing them so much it is life's greatest challenge.
The uniqueness of an only child adds to the intensity of the loss as many mother child relationships are many times closer with the child than their spouse. I know in my own life their was a reliance and connection with my own Mom that my wife and friends never understood. Since my father has been dying from Alzheimers the past 5 years my Mother always seemed like such a rock and had far more energy than all but my 3 year old so the shock is overwhelming. I have not had much time to grieve with settling my parents estate and becoming the primary caregiver for my father. Many would see this as challenging, however it gives me a chance to have more time than I had with my mother and gives me a way to honor my mother. I can only imagine how tough this has been for you as I know how I have felt the past several months and to think of this continuing for several years would be devastating. I hope you can find the strength to find support from other friends and family, support groups for those that have lost a loved one or potentially grief counseling. I don't believe time heals all wounds as they need to be dealt with or by focusing on what has been lost will be all that you feel you have.
Deepest Sympathies,
Troy