Hi Lost Guy,
Welcome and so glad you found SR! I know the tendency is to try to figure out why THEY did what they did, but it is important to focus all that energy toward YOURSELF instead. A lot of people will talk about working on their OWN recovery and you may ask yourself "What are they talking about? They aren't alcoholics?" But what happens when you are in a relationship with an addict is YOU wind up being damaged also. You have spent so many years focused on the other person's problems, you forgot that normal people focus on themselves first. Please read about codependency on this board and in literature. I recommend "Codependent No More" by Melodie Beattie. It was very eye-opening to me!
You have made a powerful decision. I commend you for realizing YOU are special and do not deserve to be unhappy in life. Try very hard to let go of the tendency to think about HER and instead allow yourself to focus on you. It is not selfish or self-absorbed, but as a codependent we tend to think that way. In truth, it is vital to recognize how much YOU have been pulled down by this relationship, and how much YOU need to recover, both for your happiness and so that you don't wind up repeating the same poor behaviors again and again in the future.