Honestly, this sounds awful.
Initially I was wondering if you're the one putting the pressure on yourself, but as I read on ... you're clearly not.
I'm assuming you're both adults and you made choices that led you here. Now you realize it's time for you to make different choices and that means your partner has to figure some stuff out (and likely so do you). That's fine.
What you and/or they might be doing right now is comparing the suffering:
Is it worse for you or them if you end the relationship? How can their problems be mitigated to come out of this even?
Unfortunately suffering is always bad and there is no point measure of meltdown where one person objectively suffers more. There's also no mitigation, you stand where you stand.
Be as gentle as you can, but with this sort of dynamic also be very clear, announce and enforce your boundaries.
In my opinion the next step is theirs to do:
Find support, reach out and explore their options. That's not for you to do, because otherwise it may always be "thanks to you" and not their own achievement. If they get this right once, they can do it again, that's a big confidence boost.
You could leave them a list of support groups or other places where they can find help, but that's as far as I would go.
It's important to understand that they're not doing this to hurt or harm you, but it's just as important to know that it's still the effect they're having and stop them right there.
Good luck!
