Sorry to hear about your experience, you did nothing wrong,assuming you didn't use anything in front of her yourself. Which I am presuming not.
I would suggest looking at support groups for SO's of addicts. Just so you can see others experiences.
You are not alone, this is a common story.
Relapse can happen for many reasons, most commonly,untreated mental health issues. Falling back into the wrong social circle. Most common amongst younger users is they just haven't fallen hard enough or far enough to genuinely want to give it up. Especially if they have family or friends who keep picking them up so they don't hit rock bottom.
You did nothing wrong, she just isn't ready to give up that life.
Look into protecting yourself should she return looking for help. Which probably means money, and be prepared for unpleasantness if you refuse.
Take the previous posters advice and take sometime with a professional to get things straight in your head.

I am an addict, I have been for 24 years. You cannot help her. She has to want it, more than anything else.

Good luck, take care of yourself.