How can one differentiate between a genuine deficiency in the number of Rak'ahs of prayer or in ablution and the whispers of Satan, especially when actual errors intertwine with these whispers?

If you are afflicted with obsessive doubts (waswasah), such that you experience doubt even once daily, then your cure is to turn away from these doubts and ignore them, even if Satan whispers to you that the matter is real and not just a mere doubt. What Allah has legislated for one afflicted with waswasah is to resist these doubts, ignore them, and not indulge in them. So, if you doubt whether you have prayed a rak'ah or not, ignore the doubt and proceed with your prayer, assuming that you have prayed it. And if you doubt whether you have washed a limb or not, act as if you have washed it and proceed with your ablution.