This will take some time to fully sink in. No one is 100% clear this soon after something so tragic. It has to be hard for you to consider this, but you have to consider divorce.

Just in case, you need proof of her cheating. If you can get this proof through couples counseling it might be worth it to go. This may sound uncouth, but you can also record her saying it with a camera without her knowledge, its perfectly legal. You might see this as a breach of her trust, but its literally nothing in comparison to her breach of yours. If she decides to deny it in court, it will be your word against hers, and you will regret not having proof. If you dont get divorced, it wont matter and is a complete non-issue.

She specifically told you that she didnt want to tell you because she would rather lie than hurt your feelings. Im sure shes also told you that it wont happen again. Given these two facts, its almost certain that if it does happen again she wont even think about telling you since it would be even worst than the first time.

Even if counseling goes amazing, part of you will always wonder if she started being faithful or if shes lying again. That isnt fair for you. Its something you might deal with until the day you die, always wondering if you shouldve found someone better. Someone who actually loved you enough to never consider cheating. If you have children, this could create an inherent tension between their parents. Thats not good for their upbringing and would not fair to them either.