Resentment

I read that resentment is most powerful when it is felt toward someone whom the individual is close to or intimate with. 

Resentment is an emotionally debilitating condition that, when unresolved, can have a number of negative results on the person experiencing it, including touchiness or edginess when thinking of the person resented, denial of anger against this person, and provocation or anger arousal when this person is recognized positively. By contrast, resentment does not have any direct negative effects on the person resented, save for the deterioration of the relationship involved.Resentful feelings are dangerous to live with and need to be dealt with. 

Psychologist James J. Messina recommends five steps to facing and resolving resentful feelings. (1) Identify the source of the resentful feelings and what it is the person did to evoke these feelings, (2) develop a new way of looking at past, present and future life, including how resentment has affected life and how letting go of resentment can improve the future, (3) write a letter to the source of the resentment, listing offenses and explaining the circumstances, then forgive and let go of the offenses (but do not send the letter), (4) visualize a future without the negative impact of resentment, and (5) if resentful feelings still linger, return to Step 1 and begin again.

Research suggests constant bitterness can actually have negative effects on our physical health.

Moving on may be a better alternative to bitterness. People can reduce stress by letting go of unattainable goals, which may actually decrease levels of cortisol. Getting rid of grudges may also reduce anxiety and lower blood pressure; in some people, forgiveness may improve cardiovascular health.