undermine somebody's morale
weaken somebody's resolve

But she allowed none of that to weaken her resolve.

You did not feel compassion, it weakened your own resolve.

Those might weaken his resolve, and he could not afford that.

"That's why the rebels must be made to understand they haven't weakened our resolve."

One said, We wonder if too long in a solid body weakens resolve.

It seems to have weakened her resolve against me.

Little changed, but that has not weakened her resolve.

He would not- could not- allow the moment to weaken his resolve.

But recently, something happened that threatened to weaken Field's resolve, not to mention her hopes for what she calls "a great third act."

Alex was pretty amazed herself, but refused to weaken her resolve.