Stephen,
I remember a lecturer from several years ago who said that when worry comes "push it away with both hands" as he put his hands up as though he were pushing an annoying person out of the way. He meant that you should not let problems take over your mind. I have found several ways to overcome the "what ifs." Joy is the best remedy. Rejoice in what is posative in your life. Make a celebration, even if it is only sharing a good joke or a special treat with someone else. Find something utterly beautiful or silly to concentrate on. Make faces in the mirror. Do busy work like making a list of books to read or a task you have been avoiding and probably always will. In other words,treat the "what ifs" like a jack in the box, just keep pushing them away any way you can. Or do a "Scarlet O'Hara" and just tell your self you'll worry about it "tomorrow."