There are some people that I work with quite well on a day-to-day basis that I basically do not trust or only trust to a point. My distrust is based on observation of their behavior over time. I've learned to trust my instincts and pay attention to the red flags--those little twinges and bells that go off when someone is just a little too charming, ingratiating, or flattering, for instance.
Trusting myself enough to know someone else is not to be trusted is something that took me the better part of 30 years to learn.
Being careful about who I trust basically means managing my emotional boundaries very carefully. There are some people I get along with very well who I basically view as "lost causes"--if they ever change their ways, it won't be because of me or anything I do or say.
As a consequence, I generally do not open my heart to a person with a record of breaking other people's hearts, or align myself socially and politically with a person who seems confused or double-minded about where they stand. I don't presume that the Spirit will protect me when I've failed to take some basic steps to protect myself. Nor do I presume that faith and good intentions can ever over-ride another person's freedom to choose evil over good.
Some folks think that if you've forgiven someone for the injury they once perpetrated, that means you've completely forgotten what happened and shouldn't think about guarding yourself against a similar assault. WRONG, imo. Forgiving someone (to me) means you have no intention of getting even or striking back. You have rejected the possibility of revenge and are open to reconcilliation.
But forgetting? That's just plain foolish. I only begin to forget the wrong someone did me when I see a consistent pattern of changed behavior. Not forgetting an injury doesn't mean I fester and stew over the memory, only that I'm taking care not to get bitten a second (or third or fourth or seventy-seventh) time. Or bitten the first time by someone who has left tooth marks on the folks around me.