What is the status of a Muslim girl who doubts the existence of Allah Almighty and performs acts of worship such as prayer and fasting? Is she considered a believer or a disbeliever?

From the sister's previous questions, it is clear that she suffers from whispers (waswasah) concerning matters of belief, and she should not call that doubt in the existence of Allah, because doubt is disbelief that expels one from the fold of Islam. Her fear of misguidance and entering Hellfire, and her adherence to her religious obligations, are evidence of her faith.