My head is full of everything I haven't done yet.

A head full of undone things makes sitting down feel irresponsible. The reply to Martha names her twice, which in that idiom is tenderness rather than rebuke. The work was never dismissed. She was told she was troubled, which is a different diagnosis.

The conversation

The child: “Daddy God, my head is full of everything I haven't done yet.”

God: “Martha's was too. A house full of guests and no one helping.”

The child: “Her sister just sat there?”

God: “Mary sat and listened. Martha noticed, and said so.”

God: “'Martha, Martha, thou art careful and troubled about many things.'”

The child: “That sounds like a telling-off.”

God: “He said her name twice. That's tenderness, not scolding.”

God: “He didn't say the work didn't matter. He said she was troubled.”

The child: “So what was Mary doing right?”

God: “She'd noticed I was in the room. The meal could wait an hour.”

The child: “The list doesn't stop though.”

God: “It never does. That's why you sit down before it's finished.”

The child: “Then I'll sit for a bit.”

God: “Come. The list will keep.”

"And Jesus answered and said unto her, Martha, Martha, thou art careful and troubled about many things:" Luke 10:41, KJV