Exactly. Except that I would have thought that a lot of the time it in fact it ought to be reasonably clear which applies.

The quoted sentiment seems to be "when you realise you used to think something different [and implicitly: that the new thought is, on considering both, obviously the correct one] you shouldn't attempt to justify the old thought, but to accept that you changed".

Conversely, the situation you describe is I think generally when a temporary emotional response overrides a previously clear decision (and implicitly that the previous decision was likely to be correct).

However, once you get as far as diffing the old-you and new-you attitudes, it seems likely that which is appropriate will be obvious.