How can I help my Girlfriend, please.

For a bit of background, in the 8th grade I still had lots to learn about love, I ended up cheating on my girlfriend with my ex, and pornography. It has severely messed her up as well as everything in this relationship, and I have been doing nothing but picking up after the broken pieces every single day since, of course I know this is my fault, I feel like complete shit due to the fact that she had chosen to stay with me, but now we have been together for 4 years now, I’m going into senior year in high school , but I feel very dragged down by this relationship, not that I want it to end, I just want to fix it. I do absolutely everything try thing I can for her, I try so hard ever since I felt what it was like to lose her, anything she asks is done before she even knows it, and I do it without question, on the other hand, whether its me even asking if she can lower her voice or be a bit more caring, she will just yell and say “I’m not yelling” whilst also saying “I don’t care” and “you cheated” she has been diagnosed with a few mental issues throughout our relationship and has gotten meds, but those just tend to make her more sleepy, they don’t help her find meaning or care about anything, she just hates everything, no therapy, no help from me or anyone, it feels like I’m just watching her fade away and Im not going to be the one to let it happen, her parents are a bit messed up as well and she gets no love or attention from them meaning it genuinely feels like im a guardian to her in a way, she will yell at me to leave her house then threaten to cut herself when I do, she will beat me occasionally, but I let it happen because in the end, well it was me who was the root of it all, I just really, really would appreciate someone to talk to and or find help for me or her because I have been feeling really alone through this with no one to talk to, and the only response from the only one I love is “I don’t care I hate you”. I will literally do anything to have my girlfriend live happy and normal again, whether it’s with me or not.