What was IT?

Tell me what IT was,
  that you saw
  and then knew.

Something about me was exposed
that changed how you thought of me.
IT did not set right with you,
and everything–every, single, thing changed.
Yet you never talked with me about IT
whatever IT was.
IT was unspeakable.

Something cosmic, unique
was happening between us
and then there was that fatal insight.
It has been a slow halting death
with random spurts of hopeful re-tries.
Still the end seems near now,
because IT was unforgivable.

Tell me what IT was,
  that you saw
  and then knew,

so that I may hide IT more carefully.