Relationship friction, failures and mistakes, duty and responsibility, moral dilemmas and temptations

The character being unwilling to reconnect with a parent who abandoned them as a child
Reconciling with an unfaithful spouse but not being able to stop thinking about them with someone else
Sibling rivalry as adolescents causing trust issues that the character carries into adulthood
Holding a grudge against a high school friend who betrayed the character long ago
A parent who once played favorites with her children trying to act like it didn't happen and the character being unable to forgive her
Not being able to forgive a stranger whose irresponsibility or selfishness changed the character's life significantly
Suffering an assault or abuse by a known person and being expected to let it go so everyone can move on

Tensions rising from the character making snide comments about the offender at social gatherings, family events, or work functions
Arguments between the character and the offender
The character passing up beneficial opportunities (such as a great job or an all-expense paid trip) because it comes from the person they can't forgive 
Having to endure token apologies 
The person trying to buy the character's forgiveness through gifts or grand gestures 
Awkward interactions with estranged family members at holiday gatherings
Family members taking sides in the dispute
Friends growing tired of the character complaining about the person they can't forgive
Having to socialize with a high school bully at a reunion, wedding, or funeral
Needing to attend emotionally challenging counseling sessions to address one's bitterness