Your G/F is lucky to have you and lucky that you care so much.
Im the mother of a 21 year old and If she were my daughter I would insist of immediate care for her.Doctor/emergency whatever.
She needs you to step up and make the decisions for her as she may not able to do it for herself.Drugs and depression is a VERY bad mix.....believe me I know.
With the right help things CAN get better.
Good luck and read as much about depression etc as you can,that will really help too.