I love my therapist - she has awesome suggestions all the time for practical ways to deal with issues in your life. I think everyone should have therapy at some point in their life to examine issues that have been problematic and also to have an un-biased confidant with resources to help you.
If you're like me, you want to see the good in people and you often do this to your own detriment. Missing an ex we were surely in love with is a great example... quite often we romanticize the good parts of a lost relationship instead of accurately remembering the good and the bad. I also have a tendency to romanticize people as being "better" for me than they actually are. Often it's very difficult to let go of someone you have attached to if you're incorrectly remembering the past.
This is where the 'jerk card' comes in. Now I'd say on the whole usually people need reminders about the good in each other but it's the opposite for me. Someone has been a jerk to me, I usually forgive & forget without analyzing, leaving that possibility open to be hurt by the jerk again.
If this happens to you too, make a card listing all of the negative & damaging personality traits of this person & title it "(name of person) is a Jerk." Keep this card with you in your purse or wallet where you can easily access it. Examples: manipulative, breaks promises, doesn't listen, emotional con artist, overly narcissistic, not trustworthy, etc... You might realize after listing all the negative traits that someone you considered a friend, in reality is NOTHING like a friend should be.
Now at some point (after all the good & bad aspects of this person are weighed accurately in your mind) you'll need to move past the pain of what this person did you, make peace with it, resolve it won't happen again and toss the card. Because there's no point in continually re-living the hurt and picking at that wound.
But to get to that point, this can be a helpful technique of self-protection.