The law of resentment

“For every Action there is an Equal and Opposite Reaction.”-Newton's 3rd Law.

The logical view of Resentment would be that 1 person is objectively much more in the right, and therefore the other is in the wrong. Aka, the person in the Right has a justified High resentment level, and the person who has objectively been doing Wrong should have a Low resentment level.
 
I visualize the Scales of Justice or a Seesaw:
Resentment is High on one side, and the person who did wrong would have Low Resentment on the other.

Trust  is a very similar situation:
The Person who betrayed my Trust should still have high Trust in me. But my Trust in them would logically be low.

Right or Wrong, this is not how Resentment or Trust works.
 …This is also why “Resentment eventually will destroy any relationship.”
[See "Caretaking and Emotional Caretaking" for a refresher]

The Law of Resentment is simple. It is Illogical, but it is incredibly simple:
However resentful I am towards a person, that person is at least as resentful towards me.

This is also why we falsely think a relationship can handle more resentment that it can.
"Resentment cannot be clearly gauged: we may think we are fine one day, and the next we are furious; we never know what will be the last straw that leads to the end of the relationship ('detaching out of anger')."

It does not matter that I just helped my friend pack up their house, moved all their boxes down two flights of stairs, loaded the Uhaul, then drove with them in the Uhaul across town in rush hour traffic, unloaded the truck, AND THEY DID NOT EVEN THANK ME.
...I clearly deserve to be resentful.

According to the Law of Resentment, this does not matter. However resentful I am, for whatever reason, they are basically as resentful at me (for some illogical reason).

For many years, I have prepared people (usually teens) who are trying to build back trust that, "Right or wrong, It will always take longer to build back Trust than you think it should." And I have observed countless times the person working very hard to build back trust get frustrated that Trust in them is not coming back; and in turn, stop trusting the person(s) they were working so hard to rebuild Trust with.

As "Caretaking and Emotional Caretaking" says, Resentment is to be avoided.
The best strategy to manage Resentment is to keep it as low as possible by addressing it as soon as it comes up.

Being honest with ourselves about the illogical nature of "The Law of Resentment" is a great thing to keep in mind to help talk ourselves into doing the the harder thing and Have an Honest and Direct Conversation with someone.