Melissa: I am so sorry for your loss and agree with all who have said that it is so important for you to be gentle with yourself at this difficult time. It is absolutely not your fault that your mother died by suicide. NOT your fault.
Nor do I believe suicide to be a selfish act. It is the product of pain that the person feels incapable of resolving, and is often an impulsive act--especially if the person was intoxicated at the time of death. It is terrible to think that a loved one was in such pain, but it's important to remember that we did not cause and could not cure that pain.
Please take care of yourself.