I know this from experience. I recently came from a tournament where I was beating a master and lost because of time pressure. It's always hard. Based on my experience, I would like to share some tips with you that may help you if you were ever in this situation.
1. Calm down. It is always important to calm down and breathe. Losing to a higher- or lower-rated player is hard but you learn from your losses. Therefore, after a loss, ask your opponent what you could have done better. It is always good to increase your knowledge because eventually it will make you a better player. Remember that you are constantly learning. Even GMs learn new ideas and concepts to this wonderful game! Also, remember sportsmanship! It is always good to treat your opponents the way you want to be treated.
2. Shake it off. When you lose the best thing to do is shake it off. How do you shake it off? You shake it off by determining not to concentrate on your loss and focus instead on the next round. Understand that if you go into the next round feeling bad your chances of losing again are higher, because you are still concentrated on your previous mistake. It is best to let it go and concentrate on fixing your errors later. Remember constantly winning doesn't necessarily make you a champion; it's learning how to get up from defeats that defines you as a champion.
3. Have fun! It's never fun losing but enjoy learning making new friends! You never know, they might become your best friend for life!
Remember chess is an amazing game! Enjoy, learn and have fun! I hope you learn from my experiences, and incorporate them in your tournament play!
Watch my game against this ChessKid "regular". I played the Nimzo-Indian and won a rook for a knight. Eventually, he resigned. Can you figure out what my winning plan would have been?
In this one, my opponent played the Englund Gambit, which was just analyzed in FunMasterMike's recent ChessKid TV show (archive coming soon). Luckily I won a rook for a minor piece again and I was able to win.