It generally takes effort to truly hold a grudge, and it’s because of that effort that grudges are bad for you. At the same time, however, you should not completely trust someone that you cut out of your life. To do so is to say that you had no reason to in the first place, and that that person is not capable of hurting you again which does not serve to help you either. They may never do that thing again, but from their point of view it was never out of the question in the first place (at least not so much so as to make it not worth doing). It’s because of this, that it’s more important to be careful of who you associate yourself with in the first place, than to be wrathful of those with whom you no longer associate.