Absolutely. Yes, it does depend on the situation, but on the whole if someone does something I do not like, that is it. I do not forgive or forget, I will "write the person off". If I have to interact with this person (work, for example), I will be civil if necessary, but probably do not do very well to hide my contempt for them.
As opposed to it being a waste of energy, or effort, I see it as the opposite. If that person if effectively "written off", it's one less person I no longer have to worry about being in my life, one less person to make the effort to pretend to give a sh!t about, one less person to not have to pretend to acknowledge, or pretend to be interested in their feelings or interests.
This is all sounding incredibly harsh. I am of course talking about if someone does something I view as unforgiveable, in context and I'm not of course talking about family, my other half, or children; more a colleague or someone that never really mattered in the first place...
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"You're entitled to your wrong opinion..."