Steve, I can see you becoming Simon very easily. You are that close. That is in fact something that often happens in these obsessive types of relationships, doing whatever it takes to have the person in your life. 'can't beat em, join em'.
It can happen sooner than you think and you won't even see it happening. Especially because you are sort of gullible (I mean that in a nice way!!)
You believe her and her stories about 'cutting back' or 'thinking about recovery' when she is still actively using. It will probably be easy for her to pursuade you to use one day.