Suicide is a selfish act. No one is responsible for another person's suicide except the person who committed it. Your mother was suffering severely from demons that existed before you were born.
I had a friend of mine years ago whose brother committed suicide. I told her I was terribly sorry to hear it. She replied "I'm not, my brother has been plagued by severe depression all his life. He has never been happy. I think he's happier now."
Perhaps your mother is happier now. Perhaps she found a way to escape her demons.
Guilt is a very common emotion for those who are affected by the suicide of another person. It is not your fault. It was never your fault. The drinking, the need to protect yourself from the drinking - none of it was your fault.
That said, I am truly sorry for the situation you find yourself in. I suggest you find a grief counselor (most hospitals can help you find one) and set an appointment if you can. A counselor can help you work through this.