You are doing all you can, whatever happens it isn't your fault. I spent my entire childhood putting the mental health of my mother on my back, I found her post-suicide attempts many times, including coming home from school and walking into a kitchen covered in blood. I was lucky she was never successful, but she came very close. It's hard, but you can't 'save' anyone. Be there for them, be someone they can talk to, do what you can to be their support and help but please don't put their wellbeing solely on yourself because that just makes two people who are depressed.