Volleyball as a sport requires to the players a list of several skills or abilities, basically physicals, to be able to play the game, and usually the coaches all around the world has as a main goals for their players to be in shape and has a good physical condition for a good performance for health, and another goal that is more technical like training skills and moves for good teamwork, but they maybe do not take too serious the mental side of the players.
The psychological side of players and its training is hugely important for the good performance of the players, in other scale for the team itself a good athlete that trains all days can not do nothing in a competition if he (she) does not have confidence in what is doing, or, how this player can have a good season in any sport if does not thing and believes that he (she) can wins and do not trust in his (her) teammates?
The self-confidence is very important in a player, but it has to be promoted by the coach and their teammates, the coach has the job not only of train physically the players and teach them the moves, strategies and techniques of volleyball, but give them motivation, make that they can believe in themselves and in all the team, that they can is each other as a team and not individually and create a goal in common between them, maybe a tournament tittle or you increase the good performance of the team.
Teach them to give motivation to each others and make them learn that they are a team and if they win or lose they are doing it together, the coach also needs to know the better way of treat the team and know how control it in situations for example if they are loosing the match 2 sets by 1, what to make to keep them positive and makes them believe that the still have the chance to win even if is very hard to accomplished, and avoid the team fell down.
The main idea of this is to connect their heads with their bodies and do right what they know and have to do to win a match, make them look easier the situation, go step by step, point by point to get the price, this is all a training and you will not get it from one day to the next, obviously you will need the cooperation of all of your team and the captains to promote this thought to the rest of players.