How can one break free from the state of helplessness and loss of ambition resulting from a mistaken understanding of the concept of divine decree (Qadaa' and Qadar), especially after realizing that Allah's writing is a writing of knowledge, not coercion?

These thoughts are from the whisperings of Satan. All matters are by the decree of Allah the Almighty, and it is not permissible to abandon taking means (causes) relying on destiny, because Allah has linked causes to their effects both legislatively and by divine decree. Whoever takes the permissible means, that will lead to the occurrence of the result dependent on that means. Therefore, abandoning taking means is a defect in the Sacred Law, because it is the Sacred Law that commanded and urged them. So, you must cast away these whisperings and take the means that lead to your benefit in this world and the Hereafter, relying on Allah.