Is what befalls me of obsessive thoughts polytheism and worship of Satan, or is it merely obsessive-compulsive disorder, and does it affect the validity of my prayer?

We ask Allah to grant you healing and well-being from these whisperings (waswas). You must strive against these whisperings and turn away from them. No matter how much Satan whispers to you that you are drawing near to him or worshipping him instead of Allah, seek refuge in Allah and proceed with your worship normally, knowing that these whisperings do not harm you as long as you dislike them. Beware of neglecting worship or falling short in obedience in response to Satan's whisperings, for this is his aim. Repel him by rejecting his falsehood, and know that you have not associated partners with Allah and your worship benefits you. Repent to Allah for any shortcomings in your prayer and continue to strive against yourself until Allah removes the whisperings from you. You can consult the advice section on our website, and strive in supplication that Allah may remove what you are experiencing.