Before you can rebuild the trust in a relationship, you need to have an honest talk that leaves no question unanswered. If you need to know specifics about a situation or action, this is the time to ask. Once everything is on the table, it's time to ask for forgiveness. If you're the one who's been disappointed, it's important that you allow your partner to apologize and promise not to hurt you again. While you may have a hard time believing his sincerity, you need to give your partner the benefit of the doubt if you have any hope of rebuilding trust.
Each partner should take responsibility for the future and come to an understanding of how things will be handled if the same situation presents itself. This may mean agreeing to avoid certain scenarios, becoming accountable to each other or offering honest answers, even if it means giving up part of your personal freedom to ensure the other person's peace of mind. This does not have to be a permanent situation since attitudes and agreements can be adjusted as your relationship grows and trust is restored.
In situations where it seems like you are unable to communicate, therapy may help. Having a third, neutral party present can help guide the conversation toward constructive communication and find solutions, rather than freeze into accusations and blame.