If something has happened in your relationship causing the trust to waiver, you will have many things to work through.
When your mate has done something that requires you to forgive, you have to forgive, REALLY forgive. Once you have worked through the issue either together or with professional counseling, and you tell them that you forgive them, you can never hold that over them again.
As an example, if your mate has had an affair and the two of you choose to work it out rather than throw the relationship away, once the problems are resolved and the forgiveness is said, it is done!
This means that you cannot stalk your mate to ensure they are where they said they would be, call or page them throughout the day, constantly ask for reaffirmation of your relationship, it means that you forgive and put the past behind you and then move on in a new, strong, and healthy relationship.
It will not be easy, but you can do it with the right help, attitude, and commitment.
101 ways of building Lasting relationship.