Does what went on in the questioner's mind – linking the act of entering pornographic websites to his wife, then performing the act – count as an action that triggers an oath, or is it merely a satanic whisper that is not binding except by utterance or action? And what is the action mentioned in the honorable Prophetic tradition that makes inner thoughts binding?

There is no accountability in Islamic law for self-talk (حديث النفس). So, if a man divorces in his mind without uttering the words, his divorce does not take effect. The word "action" (عمل) in the Hadith refers to an action that corresponds to what one has thought about, not just any action. What is happening to you are merely whisperings (waswas), so do not pay attention to them. Watching pornography is a severe affliction and widespread evil. You have done well by repenting whenever you fall into it. A person must repent sincerely even if the sin is repeated. You must strive for chastity, avoid incitements to lust, and work on everything that helps you preserve yourself, such as fasting and associating with righteous company. As much as possible, have your family live with you to help yourself and your family remain chaste.