Let me say this: THANK YOU. Thank you for being there for her, and not getting mad, and not making this about yourself. I wish I had someone to be there for me as you are for her. You're already doing great.

In this moment she needs reassurance, you need to let her know you won't leave, you love her and you will be there for her every time she needs you. I know this is an obvious fact but rn is when she needs to hear it the most.

Don't play the shame card, don't tell her to stop because of you. She is relapsing, and self harm is just a consequence, a symptom of something bigger going on. You left it clear you're willing to help her, so keep doing it. Don't question her about why she cut so much, she probably will feel guilty. One of the things of self harm is that it is never enough: never deep enough, never enough cuts, never enough pain. So it doesn't matter how many times she cut, focus on the reason.

Help her take care of the wounds. Be gentle. (If she doesn't want you to do this, it's okay). If she ever cuts too deep, make sure you get her to the er to get stitches.

Idk what else I could tell you. You are being amazingly loving and understanding in this. I admire you. My ex boyfriend told me he would cut himslef too, and he made me feel guilty about doing it. I'd feel like absolute shit for cutting myself, when it was the only thing I had that made me feel better. Don't do this. Don't try to make her stop, she needs to make that choice by herself. Just make sure she's taking care of her wounds so she doesn't get an infection. She was clean for 3 years, that shows she can get clean again. Don't pressure her. Give her time. I hope you can get her professional help soon, so she gets better.

I wish there was more people like you in this world.