Resentment - A persistent feeling of anger because you are forced to accept a perceived injustice or something you do not like.
Someone who is consistently resentful often continually plays small injustices over and over in their head.
You can be resentful at a person or at a larger system.
It is most often triggered by relationships with inequality: Boss/employee Parent/child
It can also be triggered in equal status but unequal contributions or sacrifices.