Is the phrase "I love you sincerely" considered a euphemism for divorce, and is doubt about the intention of the euphemism considered a doubt that does not remove certainty?

As long as you are afflicted by waswasah (obsessive whispering), your divorce does not take effect, even if you utter the clear words of divorce. Your wife is still under your marital bond. Therefore, turn away from these obsessive thoughts. Make sure to perform ruqyah (Islamic exorcism) on yourself with the lawful ruqyah. Your frequent questioning about the rulings of the words you utter under the influence of waswasah will only strengthen it within you.