My girlfriend breaks up with me everytime we argue because she can't control her emotions because of her mental problems?

She has severe depression and she's bipolar. Every time we argue she breaks up with me and says things to me I can't handle sometimes. After we break up or after the argument she apologizes and says that it was because of her mental illnesses. I love her a lot but every time we break up I always feel like it's going to be the last time talking to her and it's taking a toll on me mentally. It's gotten to a point where I don't know if she's coming back anymore and I don't even cry that she's leaving me. I hurt her a lot but not on purpose, she's really sensitive and overthinks a lot. She cuts herself sometimes when I hurt her even though I'm there for her. I hate seeing her hurt, mentally and physically, I often think to myself that leaving her is the best option to be happy but she says she can't be happy without me and it leaves me thinking whether or not I should leaver her. We've been together for almost 3 years and it's been like this since after the first year. What should I do?
My girlfriend breaks up with me everytime we argue because she can't control her emotions because of her mental problems?
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