May I recommend a book?
Passionate Marriage: Keeping Love and Intimacy Alive in Committed Relationships by David Schnarch

Gordon, you haven't said whether it is you or your partner who has the stronger libido. If it is your partner and you feel yourself just not desiring sex anymore, the first stop might be your family doctor. You may have something physically wrong, a major disease, or perhaps are depressed. Please get yourself checked out today. Also, as men age, as I'm sure you know, many have trouble "sustaining." Viagra etc may be just the ticket. You and your partner perhaps might also benefit from couples counseling. I think many sexual problems for couples have their origins in relationship difficulties. If there is a lack of emotional intimacy, it can be hard to have physical intimacy. Good luck.