I know the cycle is same...

The abuser becomes increasingly arrogant and bullies you

but when the really hard time comes in her life, she becomes nice because she needs you (i.e. she has no choice)

You see her sulking and feel bad for her and reach out to her

And as soon as things start getting better (such as recovering from an illness),

she is back to her old, arrogant-abusive self again, bullying you.

And you regret giving her access to you once again by reaching out to her.

Yeah, I've been there more than once.

And yet, I see her going through hard time again, and feel sorry for her again...

...but I need to protect myself.

I was listening to a story about a woman who rescued a poor snake in the frost, and warmed him up by the fireplace. But when he woke up, he bit her, returning harm for her kindness.

I don't think this is a true story but a metaphor. Abusers are like that. They seem so harmless when they are down.

I need to remind myself that she will bite again. Please help me to snap out of my sympathy mode for her, as I am struggling.