Welcome...I believe that you have made the right decision, he has a disease that has no cure, it is only a matter of whether he is clean/sober and working a strong recovery program for life or not. This is a progressive disease, left untreated it will get worse.
You are so young and it would appear that this is a rebound relationship, I would back off and get yourself back together.
I would also be tested for STD's, an IV user can spread all kinds of nasty diseases around.
Take some time to read the stickeys at the top of this forum and cynical one's blogs.
The book Codependent No More by Melodie Beattie might also be of help to you. You have chosen a career (nursing) that is one of a caretaker, most of the nurses I know struggle with codependency issues.
Keep posting, it will help. Sending support your way.