Only the right criticism is important
There are two types of criticisms: the one which helps and the one which wants to destroy. You can easily distinguish the two because while one does inspire you to correct the other simply wants you to stop and never go back. There are tons of toxic people out there, who don’t like to see anyone else happy. There is no explanation for this behavior, neither can it be changed by you. The one thing you can do is to assess which criticism tries to help you and cease to pay attention to any criticism which clearly comes from bad will.
Listen to your teacher and try to choose a work group where the atmosphere is good and it’s constructive. You will easily feel this, because once you are unhappy at a place, despite liking what you do, it means there is one or more toxic people who try to destroy others’ motivation. Therefore pay no attention to bad criticism and switch group if necessary, so that you can go on drawing and creating happily and peacefully.