I agree on allowing the grieving process to happen naturally. But I also believe you can begin to focus on the positives this new relating style will bring to your life. The level of honesty and openness you may achieve with your husband can only help you both grow as a couple and as individuals. The choice to embrace your husband's new love relationship will also expand your mind and your heart if you allow it. and the personal growth you may experience will astound you.
Grieve as long as you must but keep your heart and mind open about the positive aspects of this relating style and focus on your own personal growth.