When I say sh, I press the middle part of my tongue upwards against the top of my mouth. When I say ch, I press the front of my tongue against the top of my mouth. In both cases, the corners of my lips are closed.
I would email your teacher and let her know you have a difficulty, but you have to stop hating yourself. The more you do this, the more the problem will grow inside you. The more you try to conceal it from people, the bigger it will seem to you when they find out. And these things often seem bigger than they are. I have always struggled with the mechanics of writing- I cannot hold a pen properly or write clearly, which caused me a lot of trouble at school, though I only use capital letters nowadays. Now I couldn't care less. Some people do things a little differently. Some people cannot pronounce the letter r, other have lisps. It may be a bit embarrassing the first time people hear it, but you should not let it dominate your life. It really is more about what you say than the way you say it- if the group will benefit from what you have to say, then say it.
Student or Learner