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You've re listed your question and I will try to assist-
1) you are admitting that the cover up and the fact that this relationship was interracial were the issues
One possibility is that you may be wondering now if this is the only time your wife had withheld information (and therefore working yourself up as to thinking about what else she may have not been honest) If this is the case, then the trust issue is a major one in any relationship.
If she had been honest and a decent wife for these 17 years and had decided to keep something private (which she has the right to do so,) you may want to find out what had made her not want to share this (Is it because of your reaction) or (is it because of something else ie. she does not trust you, she does not want to feel judged, etc)
If this is just one incident in her life (and one thing she had kept private during your marriage, then, you would want to figure out why is this becoming such a major issue to the point of losing the marriage and all that time and energy and love the two of you had shared (and if it is worth it) It would be like giving power to this occurrence that took place many years ago and let it break what the two of you had built.