"Bad opening"? Well, if you don't have much time to study, you might try learning the Colle (Koltanowski or Zukertort) for White and something like the Scandinavian (Center Counter) for Black.
"Silly mistakes"? Immediately after he moves, ask yourself "What's the threat?" Immediately before you move, ask yourself the Marithon Man question: "Is it safe?"
"Rush the clock"? Divide up your available time into easier-to-monitor segments. For example, for your particular tournament's time limits, calculate out where your clock should be after 10 moves, 20 moves, etc. If you're moving too fast, take a deep breath, and double-check for silly mistakes, as above.
I am an averagely good chess player.However the reason that I lose so many games is either:
If you have any tips they would be grateful as I have a tournament coming up.
Thank you,