I can only speak for myself, but leaving was hard for me for many reasons. Probably one of the biggest was that I saw myself as a failure. I was convinced that I should be enough for him to stop his self-destruction. When he wouldn't or couldn't, I just knew that it was because I wasn't good enough, beautiful enough, or smart enough. I was a failure.
I needed help to change my thinking. Help to separate myself from him. Therapy changed my life. It taught me that I am good and worthy, and being unable to change another person does not mean I'm not.
Just my experience.
L