I have ridden horse for 10 years. My concussion was caused by a riding accident over a year ago. Most people have no understanding of how a concussion causes serious or subtle damage to numerous parts of your brain. They so often do not comprehend how even small changes to the functioning of different parts of your brain can interact and create big problems. For myself, I know that an important competition would be potentially disastrous. Here's why:
high stakes = stress = impaired and slowed thinking
loud noises = stress = more of same
bright overhead lights = stress = more
small changes in coordination of left and right hands = could mis-cue horse = dangerous
small changes in balance = very dangerous when riding a horse
stress + stress + stress = slowed reaction time = very dangerous in some situations
get tired much more quickly which causes all of the above to get much worse at an alarmingly fast rate

Don't listen to people who don't understand concussions. They can give you bad advice. "Get back up on the horse" is a classic one and some of the worse advice someone could give you. This is the time for caution. Riding requires excellent balance, coordination and timing. Any problem in these areas could put you at serious risk of another concussion. A second concussion is very, very dangerous! When it comes to recovery, think in terms of months or even a year or two, not weeks.