Don't get too bummed. Try your beer while reading the scoresheets and see of you agree. If you taste what they taste and agree, then figure out how to fix it and keep it in mind next time you brew.
If you don't agree with the scoresheet, ignore it and send the beer off to another competition. If you get several sets of scoresheets back and they're all similar, then you know you have a problem - even if you don't taste the flaws.
Most of all - don't take low scores or criticism too personally. I know it can be difficult to hear something you spent lots of time and effort doing isn't as great as you thought. Use the feedback to learn and grow as a brewer and before you know it you'll be raking in the medals.