What is the remedy for repeating the opening Takbir or the verses of Al-Fatiha in prayer due to obsessive-compulsive disorder (waswas), and the feeling of inadequacy or disbelief if not repeated, which leads to difficulty in prayer?

What you are suffering from is the disease of waswasah (obsessive whispering), which is one of the greatest ailments. To rid yourself of it, you must completely disregard it and pay no attention to it. So, when you enter your prayer, say the takbir (Allahu Akbar) once and pay no heed to Satan's doubts. Proceed with your worship diligently and actively, for your takbirat al-ihram and your recitation of Surat al-Fatiha with contemplation and humility are sufficient. Repeating them is a way of pleasing Satan. Following these pieces of advice is the path to healing and salvation.