Hi ExCordis. I am glad you are going to a therapist. The important question is why you find that kind of person attractive in the first place...
I went out with someone like that and it is heartbreaking to see them prefer alcohol and start changing for the worse. But it is more heartbreaking to see someone who can be healthy,loving and productive to lose her life and health as well.
You have no options regarding what others will do but your life is yours and as a wonderful woman here told me "you are still writing your own story".