I am the "next girl" in my relationship, and the one before was the "next girl" from his relationship prior to that. I personally think that two relationships are not the same, people connect in different ways, agree about certain things, disagree about certain things. I think the common aspect is the alcoholism or addiction, we all have had to deal with a certain amount of hardship because of it, but different hardship manifested in different ways.
I think it is hard to separate what parts of a relationship were because of you and him, and what parts were just because of him. The parts that were you and him, whether good or bad, will not be present in his relationship with his new gf. But the parts that were him will most likely seep into every relationship he has until he finds change for himself.