Any rationalization is post-hoc, done in support of the pre-existing (and pre-determined) goal.

As such, because it's entirely post-hoc, this leads to moving the goalposts, and any respective adjustments in rationalizations.

The primary question one must ask, in these circumstances:

Is my counterpart arguing in good faith?

If the signs point to bad faith (e.g., telltale language, gaslighting or similar methods employed, etc), then rational argument is not their currency.