When you realize that it is much more pleasant to forgive than to let hate and anger poison the quality of your existence, it becomes possible to forgive. Forgiveness does not mean you trust someone who is not trustworthy, or that you allow people to harm others or yourself with impunity, or that you allow others to treat you like a doormat. Forgiveness means you let go of the unpleasant feelings that poison the quality of your existence. You can take appropriate measures against people who might harm others, but you do it out of love not hate or anger.
Forgiveness leads to non-attachment. When you understand the power of forgiveness, you will want to forgive everything and everyone because it results in a more pleasant existence. At the root of all attachments is liking and disliking. When you want something you don't have or don't like something you do have, you blame fate, the universe, yourself, God, or some such object of blame. When you forgive that object of blame, you let go of your anger and desire and you develop non-attachment.
To experience the truth of this, to develop forgiveness and non-attachment, try meditating daily. Periodically reminding yourself, "Forgive everything", with the understanding explained above can also help.