Drugs and alcohol can definitely alter a person's personality and behaviour. Unfortunately, all you can do now is be available and set firm boundaries.

It's great your younger child is in therapy, along with yourself.

We can't change the past. There will ALWAYS be "better" ways to do things. We can't beat ourselves up because it's not going to help or change anything.

Your eldest is on a path of their own and will learn the hard way, unfortunately. They'll have to hit rock bottom before they'll realize the significance of your role in their life. Hopefully, they'll come to that realization, sooner than later, and without too much permanent damage in between.

If you ever do find out, keep an open mind, accept any responsibilities (if any) and try to rebuild. That's all you can do, really.

I'm sorry you're going through this. Parenting is the hardest job on the planet.