Julie I can't remember if anyone mentioned, making a list? A list of all the horrible things he said and did. Add to it as things come to mind. Then when you get to thinking about what - could have been - what it was like when he was "sober" (you're right, he probably never was while with you) you can refer to that list. It serves two purposes. It helps your brain to switch gears - from the, we could have been at the beach right now etc etc - to remembering the time you asked him about the hidden bottle of whiskey and he flew in to a rage. It creates a balance, a reality check.
I believe the difference between a "regular" break up and one with an addict is - although they are truly 1 person, there are two sides. There was the man who was probably kind, loving, generous and complimentary and the horrible drunk guy that had you walking on eggshells. You didn't technically break up with the great guy he CAN be, you broke up with the alcoholic guy he is. So he's still out there, sometimes he's probably still the nice guy, but like you said, a shell of the person he was. It can cause cognitive dissonance.
While all of us have different sides to our personalities, they usually don't swing so far out in left field.
And you're right, you didn't take an oath of secrecy about his incredibly poor behaviour and drinking.