Is what one says to oneself by way of divorce whispers, or what the tongue utters secretly, considered a divorce, and is there sin in fearing that one might not utter the Shahada at the time of death?

Divorce does not occur merely by self-talk, as the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for what they whisper to themselves, as long as they do not act upon it or speak of it." Therefore, your wife remains under your marital bond. Intense fear of death and of not uttering the Shahada (declaration of faith) at the time of death, if it drives you to increase in acts of obedience and distance yourself from sins, then it is a praiseworthy fear, as long as it does not lead to despair and despondency. However, if it does not motivate acts of obedience or is a fear of despair, then it is not permissible. A Muslim should balance between fear and hope.