If you recall the wrongdoing and feel sad when the person who wronged you is doing well or if you feel glad then he is not doing well, this is resentment. But, if you are glad when the person who wronged you is doing well, if you are glad about his progress, then it is not resentment.
As for me (Saint Paisios), no matter the wrongs I suffer from others, I forget them all. I just throw the old ledger into the fire of love and burn it.
What did Christ say? “love your enemies, bless them that curse you, do good to them that hate you.” Real love within us shows when we help psychopaths and drug addicts. We see Christ in our brother. Whatever we do to comfort our brother, it is as if we do it to Christ Himself. “Inasmuch as ye have done it unto one of the least of these My brethren, ye have done it unto Me."