Well, you will be in limbo until you decide what you are going to do? You can wait until she and her parents make a decision about what your life is going to be like going forward, or you can set some boundaries.
There really isn't any excuse for nagging or yelling at a person when you are frustrated, but that's something you can work on later. Right now you are no doubt very hurt.
I can’t imagine why she’d choose to sit at her parent's home over her life.
Perhaps because you can't imagine what it's like to be an addict. She could be home with you, having you frustrated and angry, or with her parents who probably are much easier to deal with.
I didn’t handle it correctly and yelled a lot and told her to leave the home and go to her parents. I apologized and took it back - I feel awful. I am always on edge worrying she will relapse on drugs or alcohol. We patched things up from that incident. A week later however she was seemingly on something…
Imagine the shoe on the other foot. Imagine you are struggling with something and you have your loved one yelling at you and asking you to leave. I'm not saying you are wrong for asking her to leave, the situation was out of control, but I'm sure this isn't the first rodeo you two have attended. Your life with an addict is hell - so is hers.
While you might be "right" you don't get to choose her life for her, no matter how much you care for or love her.
Boundaries, what are yours? These are different to rules. For instance a rule might be - you must stop using drugs. When that doesn't happen - you get frustrated.
A boundary would be - if you continue to use drugs and don't seek treatment, we can't be in a relationship so you will have to leave (or you will have to leave as the case may be). With a boundary, there are no rules for the adult to follow. You say what your boundary is, she either goes along with it or you part, that's it. She doesn't actually need to do anything, it's all in your control. Rules are really just for children to follow, not adults.
I'm sorry if this sounds harsh. Please know I'm not trying to lay blame on you - at all. It's just that you are looking to her to solve this, only you can solve this for yourself.
A book that is often recommended here is Codependent no more, by Melody Beattie, you might find it very helpful in setting boundaries going forward.