In summary of all your remarks thus far on what makes forgiveness easy or difficult, I see that the pertinent factors may be sorted into two categories, externalities and internalities.

The EXTERNALITIES include:
*The magnitude of the offense that has triggered unforgiveness
*Being asked for forgiveness
*Evidence of others' willingness to accept forgiveness  
*Evidence of others' willingness to constructively change their ways 
*The willingness of the unforgiven person to admit his/her error
*The persistence of whatever is occasioning unforgiveness
*The degree of likelihood that another's offense will be repeated or that an unforgiven circumstance is likely to be perpetuated
*Whether the occasion of one's unforgiveness was deliberately perpetrated and personally intended, or was unintended (such as an insenstive "mistake")
*The physical, familial, social and emotional etc. proximity of those who have offended
*The degree to which it is possible to change the person and/or situation that is unforgiven

The INTERNALITIES include:
*One's ability to effectively and appropriately express one's resentment and anger
*One's sympathy/empathy for the victims of others' unforgiving behavior
*One's sympathy/empathy for the perpetrating person or triggering circumstance
*The magnitude of the pain of feeling offended, defenseless, betrayed, etc. (i.e., as Tim put it, "the gravity of the impact")
*The relative importance of the situation or person that is unforgiven, i.e., the degree to which one cares about the person or circumstance that has occasioned one's unforgiveness - in short, how much he/she/it "matters", 
*The degree to which one's self-esteem has been abused
*The degree to which it is difficult or easy to forgive oneself

Tim also spotted the crux of the unforgiveness/forgiveness conundrum in his reference to "The consequences, the repercussions, the perceptions."

At the core of every external or internal consequence and repercussion related to my feelings of unforgiveness, and at the core of my decision whether or not to be forgiving, is the matter of perception - my perception of the offensive person(s) or circumstance(s), my perception of whether forgiveness is appropriate, my perception of the consequences and repercussions of forgiving, my perception of my ability to be forgiving, and my perception of my willingness to forgive.

I tend to draw from all of this a conclusion upon which I would like to have your further comments: 

My unforgiving and forgiving feelings reside entirely within the realm of my own consciousness AS PERCEIVED by me. Neither my unforgiveness nor my forgiveness resides "out there." Accordingly, my unforgiveness is ultimately an INTERNAL issue, and any forgiveness on my part is likewise ultimately an INTERNAL response. 

In other words, forgiveness calls for a "perceptual makeover" on my part. This in turn defines the ultimate crux of all forgiveness-related issues: how easy or difficult it is for me to forgive myself.