I think that many of us invest too much of our own lives into that of our children. When that happens and there is a separation, the effect can be devastating. Fortunately, there is FaceTime, Skype etc. but you also sound lonely, which can lead to depression. So, depending on your finances and as some here have suggested, the best remedy is to engage in new activities so you keep yourself and your mind happily occupied.
You could enroll in Yoga, Art, Cooking, Sewing classes or any other hobby that you fancy. Charitable organizations are always looking for volunteers. If you are active try ballroom dancing or golf. There are also writer's clubs, gardening clubs, Chess, music clubs, book clubs, movies etc, groups for animal lovers and so much more (you can find local chapters on line). In today's world travel is also much cheaper and easier. Go on a cruise, visit a city you've never been and depending on how far away your loved ones have moved to, perhaps you'll find the time to visit them at least once every year?