I have recently started working in this field, and have met tens of people from my town alone who have got clean, so there is definitely hope.
Sadly it is crazy common for addicts to be ND. Many people can misuse drugs, but only some of them will continue to use, even though it causes them harm.
I recommend the book In the realm of Hungry Ghosts by Gabor Mate. It's an easy, enlightening read, and will enable you to understand addiction better.
My advice, like others would be to find a nar anon for yourself. You need to not feel alone on this path.
Secondly (and it sounds like you've done this) there are services that can help keep her drug using safe, hep c tests for free, hand out naloxone kits in case she starts on opioids, and they do phase 1 courses for people who are just contemplating sobriety. You can make the referral, but of course you can make her show up. The service in our area is called V.i.a.
Thirdly, she is working so she can still function and do things she enjoys. What else does she love? Crafting, swimming, hiking, cinema, crochet. Anything and everything to keep her busy and having as full a range of experiences as possible.
Scientific explanations help many addicts. Addiction alters the brain. It's helpful to think of the addiction as 'it': the primitive part of our brain that tells us we're hungry, tells us we have sexual desires, and tells us what we crave. She is so much more than just that part of the brain, but 'it' may override her. Understanding this can be helpful.
I'm so sorry you are going through this. I also suggest a GP appointment so they can keep up to date with her other meds.