utility vs truth...
depends on your theory of truth.
the 'pragmatic theory of truth' considers that something is true if and only if it is useful.
but most people consider that truth and utility are two different things that should be kept seperate.

i remember attending a class on 'cognitive behaviour therapy'. the lecturer asked what you would do if someone said they believed they had no friends.

one person launched into an analysis of how you would get the client to change that belief...

i said that before you launch into changing it you should assess it for truth.

it may well be the case that the person has no friends. in which case... it would seem to be more profitable to work on things they could do in order to meet some people in order to make friends.

i think... there are probably a variety of beliefs that while being true are unprofitable (not so useful) to dwell on. that being said, trying to change those beliefs would seem to be an exercise in self-deception where someone would be encouraging someone to lose their grip on reality...

i can't see how that would be either useful or true...

and it would seem to me to undermine the clients view of the world which would be counter-productive with respect to self-esteem etc.

regarding the 'forgotten' belief...
if it doesn't recur then the 'problem' would seem to have vanished...
if it does recur then it can be looked at...

sometimes the hardest thing is figuring out a more useful alternative...

and beating yourself up for holding 'irrational' or 'unuseful' beliefs... is only going to lead to pain...

regarding utility...
if you focus on the belief...
how do you feel in response?
what kinds of things do you have urges to do / what kinds of things are you likely to do in response?
are those responses helpful or harmful to you?

i guess that would be one way of figuring out utility...