Preserve and protect your Trust by withholding it from your MLC spouse. But at the same time consider that when a person doesn't trust themself in commitment, they may instead seek superficial relationships where there will be less hurt when they or the other person wants to get out; help your MLCer trust themselves by believing in them and by offering tidbits as they progress and maintain positive progress. Trusting can create trustworthiness, but reserve complete Trust for your rebuilt marriage. Give honest Trust where it is deserved even throughout the crisis; consider that mistrust has a solid place, but that you may also act with expectation--acting as if your MLCer will betray can facilitate less trustworthy actions in your MLCer.
You don't trust me anyway, so what's it matter?
That is similar to the dieter's lament that you ate a bowl of ice cream and ruined your diet, so you might as well eat the entire container. Your lack of Trust does not give them an excuse for continuing bad behavior. Of course you are not trusting because they are not being trustworthy, but give positive credit if it is due. Offer appreciation and praise for what your MLCer does well and within boundaries or rules; ex. bringing the kids home at the agreed upon time.
Trust goes to character and an MLCer's problem is not with character, it is with fear of their true Self, addiction and being a victim of emotional blackmail. Trust the character buried within the addict and victim. Being a victim of emotional blackmail indicates a weakness. But consider what it also indicates. Emotional blackmail uses a person's guilt as a weapon against them. What does the ability to feel guilt say about character?
Trusting is risk-taking. In an early relationship trust builds slowly as the partners get to know each other. Now you know your partner; you gave your trust to that person many years ago. But they betrayed your trust and the challenge you will face is rebuilding trust for someone you know has broken and betrayed that trust. There is no Santa Clause, no Easter Bunny, no Tooth Fairy and your father did not hang the moon. This is the real world; you probably realized your spouse was not perfect. But you did not think their flaws would become so great. Giving that trust again requires you risk your heart, your vulnerability, your feelings, your emotions...on the person who stomped on them. Watch and observe your MLCers changes. Words are a start, but they often come much earlier than actions and thus they may signify progress but not completion through the tunnel.
After Bomb Drop and the discovery of infidelity, withhold complete Trust until there has been an effort to heal your relationship as well as each of you as individuals, until there has been active remorse--words are not enough--and until there has been active rebuilding of the relationship--whether it is a marital relationship or a relationship that will continue as former spouses and co-parents.