Depressed partner (37f)...when do I (42m) throw in the towel?

At what point, when dealing with a depressed partner, can you walk away without feeling like you are abandoning them?

I go back and forth on this every day. I am trying my best to be supportive, but it is seriously weighing me down and starting to affect me as well.

It's the constant negativity and self-deprecation that just makes me tired...

I want to help her, but I know only she can do it. I don't want to abandon her, especially in her time of need. But at the same time, I don't want to become depressed because of her.

What does it mean to be a good partner?