The best way to stop excessively thinking about something - either stressing or fantasizing - is to let it happen in reality. Chances are, it will never be half as bad or nearly as amazing as you have been going crazy believing in your head. Let things get out of your head and actually happen. You will feel lighter and more at ease. Of course, when it turns out to be underwhelming, you will be disappointed. But isn’t it better to know that and move on to other things, greater things that will not fail your hope and idealism?