Midlife Crisis

I have just turned 54 and guess am experiencing a midlife crisis.  I am retired, my health is very good, my marriage is great, as are my other relationships, etc…To many it seems I have a pretty perfect life and I really can’t argue with that perception.  My question then is why all of a sudden do I have such an overwhelming fear of death, of time running by too fast, too quickly?  I become overly anxious whenever I stop and think that my husband is 62 and our time together is suddenly limited.  Even as I write this I feel my heart racing just at these thoughts.  I always thought I had a pretty deep faith in God (I am a Reform Jew) but all of a sudden….?  (I am meeting with my Rabbi to talk about this but have great respect for your work on insight as well.)  Is it just me?  Is there something else I can do to get on and enjoy life?  Thank you so much!