
Weird for me to run into this, as I was the toxic person with mental health issues many years ago. But I say this from that perspective: when it comes down to it, their mental health isn't their responsibility, it is only theirs. If they are actively trying to get better any way they can, and you can handle being with them, great, stand by them. But, if they've been dealing with this for literal YEARS, with seemingly no change, and it's taking a toll on your own mental health, you have the 1/2
2/2 right to prioritize yourself, and [gently] let them go or cut them off if necessary. Also, the "threatening to hurt themselves when you're not there" is important added context. Mental illness or not, that's toxic, manipulative behavior, and isn't okay, regardless of what the other person is dealing with. Do not let this person dictate your emotions and your life.