I think by turning up the competitive nature in practice and pressure in all defensive drills. I for one have always felt that each of our drills should be a competition of some kind. Not every player will respond to competition and increased pressure in a positive way but it will help you identify the players who can handle pressure and won't fold when the going gets tough. In the old days things like loose ball drills, multiple player rebound put-back drills, and defensive knockdown drills were used to improve toughness, but in your case it's more a question of competitive toughness when challenged I believe. Take this two prone approach of increasing aggressiveness through slow daily improvement in skill execution which will lead to more confidence.
More confidence leads to aggressiveness.
And then make your practices a pressure cooker for learning to perform under pressure to increase your team's mental toughness. The Basketball Highway . All rights reserved.