When we experience pain, especially on the 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 ? You may even rate it on a scale from 0 to 10. 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.
You may also use the following technique :http://www.touch-sport.fr/blog/video-souplesse-et-mobilite/
which will help you regain comfort and mobility.
I'm sorry the videos are 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 ;)
Also, if it's possible for you, a private Touch for Health® session is a very powerful way to relieve pain, before or after a competition !
More info : http://www.touch-sport.fr/en/