Of course it hurts to think that we are so disposable. It's heartbreaking to think that all you two had together was a lie and that it is so easily gotten from someone else. The problem is, you are thinking that he is actually "thinking". He's not, he's reacting, which is different. There is no real thought process when you are abandoned by an alcoholic.
They just want to make sure that nothing impedes their numbing of their pain. So, they move on to the next "drink" because the drink you gave is empty, the next drink, and then the next drink and then the next one will always be guaranteed to be full. It has everything and nothing to do with you. The everything is that you've decided enough, the nothing is what they are looking for.
It is a blessing in disguise. I am still in the early stages myself, but I already know, it's not me, and it is me, it's me because I said, NOPE, No more. It's not me because he is not him.