It makes sense that you would have fond thoughts about someone you remember as a good person (for the first few years of your marriage), even though you know that is not the person you are dealing with now. Knowing that he is the father of your child would make this even more complicated.
It sounds to me like you are feeling guilty for romanticizing your ex and thinking about "what could have been" while you're with a new partner, who is many things your ex was not (sober, for one). I think it is normal to have "what-if" thoughts - what if my ex had stopped drinking? What if things had gone differently? It's kind of like imagining an alternate universe. As long as you're clear for yourself that it is just fantasy, that the real universe is what you've got, not an alternate dimension (and it sounds like you are very clear on that), I don't think you need to be too harsh on yourself for what your dreams are doing.
I can sympathize a bit because I too married quite young, to someone I knew in high school, and divorced after 25 years in large part because of his drinking. However, he did me the "favor" of behaving like a typical addict (deception, blame, rage outbursts, etc) so that I have never had a moment's ambivalence about the decision to leave him. I did go through (and am still to some extent going through) a mourning process for the person I thought he was, or the person he was before the addiction really took hold. I have to accept that that person is gone, almost as if he had died.
I've heard the process of separating from an alcoholic as "a death without a corpse", because the body of the person you once knew is still out there, walking and talking and occasionally even being pleasant, even though you know that the essence of that person is completely changed. It's a really complicated emotional situation, and you have my sympathy.