When trust is damaged there is a powerful, sometimes overwhelming feeling of betrayal. The betrayal can come from almost any form of dishonesty or disloyalty of one partner to the other.
When this occurs there is always despair, regret and hopefully remorse. If a couple is willing to work through the mistakes there is a good possibility that trust can be restored. Trust must be earned though; it is not something one is automatically entitled to. Therefore, the party who has breached the trust must be willing to rebuild it no matter what it takes. That requires a strong commitment, not just from the partner that breached the trust but from the other partner as well.
Apologize in earnest. It is not sufficient to simply say you are sorry for breaching the trust between the two of you. You must also begin to understand the harm and hurt you have caused. You must be able to see this from your partner's point of view and really relate to it with empathy. Unless the apology can be conveyed in a heartfelt manner, it is unlikely that healing can begin.
Clarify the WHY. There is always the question as to why something happened. If there has been an affair the hurt spouse will of course want to know why the unfaithful spouse became unfaithful. The unfaithful spouse needs to explain the behavior from his/her own point of view, expressing how the way he/she was feeling may have led to the breach. This is not the time to blame the hurt spouse in an effort to deflect taking responsibility for the breach.
Be extremely patient. All during this process there will likely be a desire from the unfaithful spouse to speed things along and get back to some sense of balance. Patience is supremely important and needs to be understood as such. Both partners need to be reminded that this will take time, courage and strength of character for trust to be restored.
Create new expectations. Partners must now decide if the rules of the relationship need to be changed. Certainly expectations will change. There will be a need to be more accountable to the hurt spouse. The hurt spouse will need more reassurance that the unfaithful spouse is committed to change. The hurt spouse will also, at some point, need to recognize the efforts of the other and begin to move toward better understanding and even forgiveness (see my Newsletter about Forgiveness).