Sometimes, resentments are permitted, on the basis that one is 'only' resenting, say, Sally, June, or Gerald.
Resentment casts longer shadows. Anyone who resembles Sally, June, or Gerald, in relation either to the feature in question or merely coincidentally, will be tarred with the same feeling. What is worse, though, is that resentment sets up a shield that blocks others from your humanity, and you realise you have immured yourself in your own turret.
It must be rooted out with identification and thence compassion and charity. That is all.