I thought I could love this broken person enough to heal him, and his whole family. Still, I simply should have known better. I feel like I failed, big time.
You didn't fail. Quite the opposite. You had the courage and foresight to pick up and leave. You realized that you can't fix someone who doesn't want to fix himself. It was a very realistic and mature decision.
Yes, you married too young and had the usual dreams of living "Happily Ever After" but it doesn't always work out that way. You didn't hang around and compound things by having children, who would truly suffer in a situation like the one you were in.
You aren't tied down. You are still young. You can live life for YOURSELF. Discover who you are and what you want and be your OWN person. You never gave yourself that chance. Now you can. Just don't rush into a NEW relationship. Take the time to live life for yourself, by yourself, before committing to another.
The worst mistake you can make is the one you don't learn from. Don't look at this as a failure. Look at this as a lesson. Study it, figure out where you went wrong and don't travel down that same path and you'll be golden!
I was in the same situation as you, depressed, alcoholic husband (as well as members of his family) who destroyed my life... but I didn't have the courage to leave until MUCH later and now I have two kids to look after, who have suffered greatly as a result of what's happened. I only wish I'd "screwed up" like you did!