When we experience pain, especially on a day of a competition, we wish for it to disappear, we try to obliterate it, usually with not much success.
If the pain is strong, it may have an impact on your posture and use of aids, and therefore compromise your performance and generate frustration. Your horse will definitely feel it !
So instead of trying to shut the pain out, give it a few minutes, try to get to KNOW your pain. How does it feel ? Is it deep ? Is it sharp ? Does it feel like it's burning ? Or more like an internal tension ? There may be an emotion related to this pain... ? How long have you had it for ? While you ask yourself these questions, hold the following points on your forehead. They're called ESR points (Emotional Stress Release). They are very powerful to relieve stress and/or pain.
I'm sorry the video is only available in French, but you'll get the general idea.
Of course, if this doesn't help, I suggest you go see a doctor and get medical treatment if necessary. A Touch for Health® private session is also a powerful way to relieve pain, especially after a fall or trauma.
More info : http://www.touch-sport.fr/en/