Hi Cass, I guess the reason so many on here seem kind of pessimistic is that many of us have tried all we could and been hurt so much by addicts that just would not quit using their drug of choice. The people here are just trying to save you from potential heartache.
If you are determined to try and work on this relationship, I suggest that you learn everything you can about addiction. Here is the link to Hazeldon treatment center-they are one of the most famous in the world and they have lots of things for friends and family to read.
http://www.hazelden.org/web/public/resforfamily.page
Reading 'codependent no more" by Melody Beattie will help with learning about your part in the relationship with someone who has a substance abuse problem. It has been in print for 20 years because it is such a classic on the subject.
Hugs, Lisa