Why you say the seeing of Oneness will make its appearance again once it is seen? ( and apparently lost again). I.mean why would it be so? Love you
For many years of continual repetition the habit has been to see duality and separation, it takes some time for this habit to drop away.
If anything is done to try to regain the seeing of Oneness or hold onto it then one is following the dictates of the separate mind again so the answer is to just let everything be as it is and Oneness will appear again, and go again apparently, until one moment it doesn’t.