What is the ruling on saying to oneself, "My wife is divorced," with the feeling of divorce occurring, but without it being an oath, knowing that the wife was menstruating on the first day?

If you did not utter what you mentioned, but rather thought about it to yourself, then there is nothing upon you, because one is not held accountable for inner thoughts. However, if you uttered a statement conditioning your wife's divorce on a specific action while you were alone, then if your wife does not perform that action, there is nothing upon you. But if she performs it, divorce occurs according to the majority of scholars, even if your wife was menstruating at the time the divorce was conditioned, because the determining factor is the time the conditioned event occurs, not the time the condition was made.