Is Believing or Doubting That One Has Committed Kufr, Kufr?

If a person believes that he committed apostasy and repents for it to Allah and if his claim is baseless has he committed apostasy? The Prophet said that if a Muslim calls another Muslim a kaafir one of the two is a kaafir, so can this also apply to oneself?If i utter words of kufr or shirk in another language even though i am told what they mean but i forget am i committing kufr or shirk?