How can a young man, whose obsessive-compulsive disorder has transformed from insulting the Messenger (peace be upon him) to doubting the existence of God and His messengers and believing atheistic thoughts, overcome these obsessions?

The questioner's fear of obsessive thoughts is an indication of his faith, because the gravity of such matters in speech is explicit faith, as stated in the noble hadith (definition). The remedy for this is to seek refuge in Allah from Satan, to refrain from dwelling on obsessive thoughts, to turn away from them, to occupy oneself and one's time, to avoid solitude, and to seek good company. If Satan reaches the point of saying, "Who created your Lord?" then let him seek refuge in Allah and stop.