Why can't I stop my mind from racing?

You cant. Most of us cant. Stop trying. Let it do what it wants. 

Occasional racing of thoughts is normal. The point that you are aware that of it is splendid. To be restless and hyperactive is second nature of the mind just like it is to be calm and quiet. Keep observing and you might be able to see where its going and which of your experiences is leading it there. Good if you can, NEVERMIND if you cant.

If not the mind, the other thing you can surely control is your breathing. Instead of trying to control your thoughts, use the same attention to control your breath. Now that your attention is switched, intentionally inhale slowly and deeply and exhale completely. Rule of thumb is that only you should be able to hear yourself breathe, no one else. The slower the breathing the lesser the noise.

Continue this for a few breaths, the less the number of breaths, the less the number of thoughts. Integrate this breathing routine in your everyday practice. Doesn’t matter when you practice, morning, noon, evening…you will start seeing the results soon. The best results are observed when you perform the routine early morning.