With serious offenses, "forgiving" prematurely may cause more damage than good. I put forgiveness in quotations here because such forgiveness might be a way to not confront a reality, and/or be so shallow that if there is contact with the offender, anger/blame/resentment/punishment seeps out in subsequent interactions and does more harm than good. Needless to say, this is not forgiveness.
When there is a serious offense, indignity or betrayal, forgiveness will likely be a process- and it should be. Depending upon the offense and the level of accountability of the offender, the person transgressed against may experience anger, confusion, deep sadness, possibly self-blame. Time will be needed.
When we forgive, we have recognized within the offender his/her inherent goodness, a part of the person that has merit, based solely on their humanity, which we all possess. Forgiveness here can happen with or without an apology and/or the accountability of the offender. It will likely be easier if there is a genuine apology and steps to make right what was wrong. However, even here, forgiving the person does not mean- or necessarily lead to-reconciliation. Forgiveness should be differentiated from reconciliation and justice (beyond the scope of this post). Briefly, reconciliation and/or justice may or may not be part of the process. When we forgive, we let go. We do not carry the weight of the offense with us. This "letting go" frees, heals, and allows our deepest sense of self to be restored in hope and inner trust.
While forgiving someone who has seriously hurt or betrayed us is never easy, understanding it both cognitively and emotionally can help the process. First, we give ourselves the gift by intellectually understanding that forgiveness is for our own freedom; it is good for us. Emotionally and behaviorally, we can engage in the process by speaking, writing, sharing, meditating, or doing something to process (and ideally transform) the pain, and to draw forth our compassion and see the offender in his/her wholeness and humanity. Secondly, in finding compassion, we must be in touch with our own shortcomings. We must know humility: we, too, have offended/hurt/"missed the mark" in our own lives. When we can sit with the part of ourselves that are whole, yet wounded, hurt and have hurt, we experience a greater fullness, integration, and wisdom to forgive, accept and heal, for ourselves, for others. In this process, a deeper refinement and inner sustenance is enlivened.