Is it obligatory for me to say the Two Testimonies of Faith every time I believe I have uttered something from the whisperings of apostasy, and what is required of me for repentance from a thought that led me to insult Allah?

The treatment for obsessive thoughts (waswas) is to sincerely turn to Allah Almighty through supplication (du'a) and remembrance (dhikr), for "Verily, in the remembrance of Allah do hearts find rest." This includes seeking forgiveness (istighfar). When one feels the whispering of Satan, known as "Khanzab" as mentioned in the hadith of Uthman ibn Abi al-'As, one should seek refuge in Allah from him and spit (lightly) to the left three times. As for the words of disbelief that a person finds within himself, these are from inner thoughts (hadith al-nafs) which Allah has overlooked so long as they are not acted upon or spoken. This is evidence of the perfection of faith for anyone who considers them grave. One should not dwell on them and should say, "I believe in Allah."