I can relate. My drug and alcohol use spiraled out of control when my sister (and only surviving blood relative) passed on June 4, 2004. I also felt as if my whole world had ended. It was a combination of not wanting to feel the pain and the guilt of "why her not me".
I have been drug and alcohol free since Aug 21, 2007. What I know is that I was blessed to have such a wonderful person in my life, she is no longer here physically but her spirit lives on in the memories I have of her. She passed, I didn't. I have developed a support system of people both in and out of the AA program and I am not alone.
It is good that you are working with your doctor and getting help. I will keep you in my thoughts and prayers.