Just sharing my abridged experience in a similar situation. Was dating a girl in high school that grew up abused (sexually and mentally) by her extremely religious family. She was smart and cute but sad all the time. When she was with me, she was like a different person, happy, adventurous, etc. When high school ended, she was struggling to get by, so we decided to live together, but of course this was a major problem with her family because of the religion stuff and they still had a hold on her. I ended up getting married at 18 because I couldn't stand the thought of losing her and her going back to being miserable/suicidal.
We were happy for one year and then everything fell apart. We fought more, she grew more distant, the responsibilities of life got in the way with us trying to juggle marriage and work and college and finances and, and, and... One day I answered the phone and it was some dude I didn't recognize. He hung up quickly after realizing he'd reached me instead of her and that was it. I confronted her and she said nothing and ran. Long story short, she became embroiled in drugs, dangerous sex, and other bad shit. The whole thing took such a toll on me that I ended up suffering from extreme anxiety attacks and depression. I lost all my friends, strained my relationship with my family, and ended up having to move 1,000 miles away just to cope. Last I saw her picture, a few years ago, she looked like one of those cautionary meth abuse pictures.
My story has a happy ending though. After years of mental illness on my side, I managed to pull through with the help of medication and have now been remarried for 12 years to an amazing woman who actually has it together and is successful. I also have 2 great kids. I had hit rock bottom, but now I'm in a better place than I ever could have imagined being in in the old relationship. It cost me though. The first half of my 20's is like a black hole. A huge chunk of my life and progression gone forever. I wasn't able to finish my undergraduate until I was 30 despite starting at 18.
I hope this story helps you a little OP. Don't be like me. You can't save someone when you're that close to the problem. She needs other outside help.