You'll win some, you lose some. In general, you'll remember your losses more then your wins. Even if you believe that the dice were clearly not in your favour, it's quite possible that considering all rolls by everyone in the entire game, your dice were about equal to your opponent.
If it helps with your peace of mind, you can use
Dice Analyzer to check this.
Something that may also help is avoiding the auto-assault button. If you only use the normal assault button, you won't get better dice, but you'll be able to stop when things start to go wrong. This way, if you are getting the worst case scenario, you can avoid losing the game to it.
Another thing to keep in mind is to always have a backup plan. Don't assume that you will win a battle, no matter how strongly it's in your favour, unless it's the only way to win. Always assume the worst case scenario, and you'll never be dissappointed.
Also,
obligatory link to an excellent post.