When does not forgiving become holding a grudge?
Yoma 87a and Rambam Hilchot Teshuvah 2 both describe requirements for doing teshuva and gaining forgiveness. According to them, the one doing teshuva must first make restitution and then, in front of witnesses, ask the wronged person for forgiveness (possibly more than once). The implication is that there is no obligation to forgive one who has not done these things.
Vayikra 19:18 forbids holding grudges.
My question is: how does one tell the difference? When does not forgiving, because the wrongdoer has not yet met the conditions, turn into holding a grudge? Is it only holding a grudge if nothing would appease you? Does holding a grudge involve a strong emotion like hating, as opposed to there just being unfinished business? Is there a time component?
For purposes of this question, assume that the wrongdoer knows that you feel wronged. This isn't about unknown slights.