From my limited experience, you can't. Your emotions have the upper hand when it comes to you. When I had my psychosomatic incident in 2010 I had no idea that I could emotionally tear myself apart like that. During the University of Dubuque's most recent meet, when our top girl had a psychosomatic event I was reminded once again how much we love the idea of controlling something. Here is the problem: you don't control much of anything. Control is an illusion.
As a coach we want to control our athletes to get the best possible performance out of them. The reality is we only have direct influence over their lives about 90 minutes a day. An athlete can quite conceivably dawdle or hammer the given workout and thus not achieve the intended stimulus. When it comes to race day we have even less hands on input into their races because we can not be there running beside them or on a bike every step of the way. From an athlete's point of view there is less control over the entire activity so the desire to do something a specific way is not quite as present. However, the desire to perform well is always present. The difference is, as a coach we can take out our frustration through the athletes, as an athlete there is no outlet once the athletic endeavor becomes a requirement instead of entertainment. Now I do not mean take out frustration by yelling at kids (although some coaches typically outside of running do) I mean by changing the workout plan for the next week or asking why someone raced a certain way. In other words, at least from my point of view, the frustration is with myself about why one of my athletes failed to do something I thought he or she was clearly capable of. So I ask myself what can I do to get that athlete to do whatever it is we are trying to do?
By asking myself what I can correct before the next go around the emotional let down of failure takes a back seat to the excitement of trying something new when I know we are already in a good basic position, despite one race. I find that looking at the global situation often leaves me with a far better emotional understanding of the situation than looking at the details. Someone can get really angry about the results of one person in one race. For example, by announcing that I am not going to even try for sub 2:19 I have already disappointed several people. However, I still intend to try to run sub 2:24, which would be over a 10 minute personal record in only seven months. That is a huge, huge improvement. Even if I fail at that goal I know that my training this cycle just blows what I have done in the past out of the water. On Sunday I did 32.5 miles with 28 of that at an average of 5:54 per mile pace. That's amazing! That is more than a marathon, faster than I ran my last marathon. Regardless of the actual outcome I have made significant progress, and I did get a half marathon PR out of it so far. By looking at my global situation (and in this case I mean just running but looking at everything is even more critical to maintaining a positive outlook) I will be happy with the entire process.
In conclusion, you can influence your emotions by the way that you view the world and circumstances of the events in your life, but you can not control them. If they want to keep your from breathing during a race or give you back pain at all hours of the day, they will. Appreciate your emotions and try not to hide them so much. I know I try to hide them often enough. My grandma is going to die soon, and I am really sad. It is strange knowing that in all likelihood we will have her funeral before Christmas because she is still alive and even yesterday was talking coherently. It is strange to think about...