My situation was quite different, but might be useful perspective.

So when my partner and I first grew close, it was because we were young and had severe enough mental health problems that we didn't feel like we could talk about it without things escalating massively (like, the police being called because of non-acute suicidal ideation kind of escalating). And somehow, inexplicably, we helped pull each other up: when she was having a bad day, I would often help to pull her out of it; when I was having a bad day, she would pull me up; and sometimes when we were both having a bad day, we somehow managed to pull each other out of it.

We were together for like 5 years, and at some point, that dynamic changed, and if I had a bad day, it would end up dragging her down, and vice versa. It just wasn't a good thing for us anymore, but I was terrified to break up with her because she had very little other than me. It was rough on me also, because I didn't really have a functional support network besides her, but I had the beginnings of one. She, however, would have to move back in with her somewhat toxic family, away from the few friends she had near where we lived. I was genuinely scared that breaking up with her would end up killing her.

But I did it in the end, because I had to. It wasn't working anymore, for either of us, and if we had persisted, it would have hurt us both. I do know that she had at least one in-patient stay at a mental health ward in the months after we broke up, so certainly it seems like things were dicey. And I know she still struggles with mental health now also. I felt tremendously guilty in the months that followed, but despite that, I didn't regret my choice az because I knew that our relationship was unsustainable. I think she knew too.

The thing is, you might feel like a safe person to your partner, but if they're having breakdowns so regularly after talking to you, that doesn't sound like you're actually functioning as their safe person. I relate to their desperation, but this doesn't sound like it's good for them either. No one person can be a substitute for an entire support network, and it sounds like you're feeling pressure to try and be that for her.

In an ideal world, you'd be able to support her to move out (if necessary) and to begin finding the support necessary to build an actual support network, but that's not always possible. You might end up in a similar situation to me, and I want to assure you that you are not responsible for her strife. Obviously we should try to be there for the people in our life if possible, and not let people suffer needlessly. But we can't be there for other people if we neglect our duty to ourselves. Whose job is it to look after you and your wellbeing? Even in a relationship with a healthier support equilibrium, that's a duty that is still primarily yours. You shouldn't set yourself on fire to keep other people warm.