It's a tough game. You can't win every board. All you can do is cut out the zeros and keep plugging away. You get bad scores for these reasons:
1. You, or partner, make a mistake in bidding, defence or play.
2. Opponents don't make a mistake in the bidding, defence or play.
3. The cards lie wrong for you, so your perfectly reasonable contract is doomed to fail.
4. The cards lie exceptionally well for you but, perfectly reasonably, you don't take advantage.
The remedies are:
1. Cut down the mistakes.
2. Bully opponents into bad contracts
3. So what? Nothing to be done.
4. When in doubt, bid up. Just in case...
The other two things are DEFENCE and DEFENCE. Giving away unnecessary overtricks is where most of the round fat zeros come from. Sometimes just holding the opponents to their contract, and no more, will gain you a good score. Letting them make a contract that should go down will always get you a bad score. There are three elements to Bridge: Bidding, Declarer play, & Defence. On average you will be Declarer 25% of the time. But you will Defend 50% of the time. Logically, then, you should work at your defence twice as hard as your declarer play. Defence is the hardest part of the game. And the hardest part of defence is the opening lead. More contracts are let through by a duff opening lead than anything else. Somewhere in all your bridge books there will be a table of the suggested card you should lead from specific holdings. If you study nothing else, study this table. You are bound to do better in the long run as a result. This is an area even top experts get wrong so don't beat yourself up about it, but do think, think and think again. Study the auction, not just your own hand, for any small clue. Above all, do not blindly lead 4th best. It is frequently a losing option. You know it makes sense...