You’re upset because you think you deserve a better role – that it’s not fair. But there’s no such thing as fair in the acting world – or anywhere, really.
The directors cast the actors they thought would do the best job in those particular roles – and this time, that wasn’t you. They may have been right, they may not. It’s like that in all acting – from school plays right through to major blockbuster movies. Even huge stars get turned down for roles.
Stop feeling slighted and do your absolute best with what you have. If you put your heart and soul into this, helping with anything you can, volunteering to build the set, make costumes, sell tickets, do make-up – anything you can, they may well consider you for a speaking role in their next performance.
Emailing the director was a very silly thing to do. What was he supposed to do? Invent a new role and give it to you? Fire another actor and give you her role? Of course not. All you did was let them know how immature you are.
If you really find it so stressful to be in the ensemble and not walk straight into a good role, acting isn’t a good hobby for you.