It is normal to find disputes upsetting. Especially if you have not been able to resolve them with the other side and you are now sitting in a room with a third party trying to work on a solution. It is not surprising that people in mediation sometimes express anger, frustration and bitterness.
Sometimes it is necessary. Sometimes people need to show the other side how angry or frustrated they are. Sometimes venting is a necessary part of working through a dispute.
While it can be helpful to express emotion, a good mediator will make sure that this does not overwhelm the process or the parties. Don’t be afraid of expressing emotion but remember that the mediation has a bigger purpose. Don’t lose sight of your goals just for the satisfaction of getting to shout at the other side. If you are worried then speak with the mediator before to see how they can help you manage your emotions.