The separate personal i is a mind impression that appears with the purpose of inviting us to awakening.
This unhappy separate personal impression continues to arise until awakening. The separate personal self impression, the I-thought and me-feeling ends upon the full realization of the Self and being established.
There is one Reality which I refer to as Self or Consciousness/Awareness. Spiritual inquiry is an inquiry into truth, into what is real.
Existing refers to a mind arising.
ISness refers to Self.
Awakening to Self is Being Self. Not a becoming but a realization of Truth.
Via contemplation and Dharmic living, existence dissolves in the Self. The mind dissolves in the heart.