Missheart - you are in a very difficult position on a very difficult road. Sadly, noone can make a decision for you and your analogy was spot on - you can lead a horse to water, but you can't make it drink.
As someone who works in the field of alcohol and other drugs, I can tell you that it is very important you have some support for yourself - whatever decision you make. I am not sure what state you are in but there is an organization in Vic called family drug help. If you google familydrughelp the website should come up. They offer support over the internet as well as in person.

I wish you all the best.

MG