Well, are these tangible triggers or is his mind just "going there" of its own accord?

There are two schools of thought on triggers - one is to avoid them at all cost and the other is to desensitize oneself to them by subjecting one to the particular triggers until the "bad feelings" associated with the trigger are no longer associated. For example, if a certain song triggers a memory, one might delete that song from their devices and make a point of never playing it again. Or - one might play it over and over on loop or only during good/calm/happy times to sort of over write the negative feelings for more positive ones or at least neutral feelings.

The issue is that we can't 100% control when/where/how triggers will present. We also can't control how long/often it will take to be subjected to a trigger before we are desensitized. Revisiting the pain/anger/resentment/loss over and over again might do more harm than good in the long run.

That said, if the triggers are something you can control, if he can talk to you about what is causing the distress, it's possible you can avoid whatever is causing the issue. For example, it's a HUGE trigger for me to have my husband's work schedule change. As a result, he works extremely hard to keep it as consistent as possible and to give me as much advance notice as possible if there are changes. He's also super transparent about when/where/why and has given me a free pass to contact his bosses about schedule changes if I ever do have concerns.

As for general "dwelling" (as my husband would label it) I would recommend a counselor who specializes in DBT (Dialectal Behavior Therapy) or even some books on mindfulness to your husband for the non specific triggers. I remember getting anxiety attacks for no real solid reason or just feeling completely helpless/hopeless regardless of how awesome my husband was being. Noticing the feelings and acknowledging them, then distracting myself toward other more fruitful thoughts was my best tool for success.

One can't "stop" thinking about something with any reasonable consistency or great effect, but one can choose to focus on something specifically. If I say "elephant" ten times and tell you not to think about an elephant - well, good luck. But if I say elephant ten times and then go into great detail describing my favorite apple pie recipe, you might start thinking of warm, gooey apple pie, and elephants be damned.

For me, I would do this by trying to stay in the moment. I might get stuck in a mental loop of negativity (thinking about a lie he'd told me, let's say...) and I just couldn't break out of it. I would replay it over and over and take it apart and put it back together. Mentally, I'd beat myself up for not being smarter or him for not being stronger or I would try to figure out what I could have done to get him to be honest... Then I would say to myself: STOP. That was then. This is NOW. He is not cheating right now - he is sleeping peacefully next to me. He is not lying now, he has gone to great lengths to be honest.

Then I would "count my blessings" so to speak by thinking of all the times he had recently told the truth or some sweet things for me or followed through with a promise.

Or - if I was too distressed or angry (mostly when I was angry) I would turn my mind to 100% non related pleasant things. The patter of rain on the roof or the smell of my favorite lotion or the feel of sudsy water under my fingertips. Sometimes the simplest of sensations can be easier to focus on and give distraction than the bigger, more abstract thoughts.

I hope your husband is able to find some relief. I'm sure it gets exhausting for you, but please please try to be patient with him. It takes a lot of practice and a lot of blind trust to let go of anger and fear that essentially are there to protest us and to make ourselves vulnerable again.

Triggers are a tough one. My suggestion is to Kerry him know your there for him and ask him what he needs from you during these moments. He may need to talk abut it, maybe he need affection, or he may need to deal with them on his own.

Support his needs as long as he's not verbally abusive during them

As allora said, there are 2 very opposite schools of thought. However there is that middle ground where you live life and let the triggers come in and work through them one by one.

There might be triggers you can avoid or recognize like a show about cheating, you can change the channel.

Everyone deals with triggers differently. The main point I'm trying to make is, ask him how you can help him.