How can the pronunciation of the letter "Ḍād" in Al-Fatiḥah be corrected to ensure the validity of prayer, what is the jurisprudential ruling in this case, and how can the difficulty of pronouncing it correctly be overcome?

The cure for obsessive thoughts is to ignore them and pay them no mind. As for the articulation point of the letter "ḍād" (ض), it is sufficient for you to pronounce it as you hear from the شيخs (scholars/reciters) without delving into precise details. You must learn Al-Fatiha correctly if possible, and your prayer is valid even if you err in the articulation point of the "ḍād" if it was not intentional. You are permitted to follow the opinion of those who hold that unintentional linguistic errors (laḥn) do not invalidate the prayer. Beware of obsessive thoughts concerning the articulation points of letters.