Codependency, putting your self worth in his reflection.
We should, really, always, look at ourselves as individuals and it never hurts to take what others say in to consideration but those are just opinions. You get to decide who you are and it doesn't depend on him, his thoughts, his actions, his personality. He doesn't get to define who you are. That sounds cliche, I know. But if you are used to codependency, thinking otherwise can be REALLY foreign. It takes work to break out of that, but it will be worth it.
He claimed that the drugs were nothing and not effecting him.
Addicts lie. This is a lie.
I just don’t understand how he can discard me so easily
He isn't discarding you, he is embracing the drugs, two different things. This actually isn't about you (it affects you of course) but really, it's about drugs.
There was so much wrong
It will help to make a list of those things, short and sweet, to refer to every time you start thinking about the "good times", because now, these weren't good times. Bad for you, bad for your kiddies. Add to the list as things come to mind. It sounds like it would be depressing, funnily enough, it can be quite freeing.