You can do nothing of value if your mental health is declining. Best thing to do is seek shelter somewhere, for some time, to heal. You may do something of value at a later time. If you allow mental health to deteriorate there may be no comeback, ever.

I disagree with your first statement. People absolutely can and do provide many things of value while the mental health is declining. In fact, being able to provide things of value is often what helps with the mental health. Don't take that away.

Seeking shelter to heal is certainly one good option, but I also encourage everyone who struggles to seek out help. That's not a fight you have to fight alone. If out break your bone, you see a specialist for fractures. If your mental health is damaged, see a specialist for mental health.

You obviously haven't been through a bad employment situation.

Consider yourself lucky.

And, to be honest, there is almost literally nothing more important than ones own personal mental health. Almost everything someone is able to achieve is built upon a foundation of their mental health. If the foundation is shaky, so will be the building.