You are certainly not alone in this, Elin. I would think you should stop feeding the obsession.

The best way to get past something is to become occupied with something else. Sports, exercise, drawing, writing, socializing - something interesting and time consuming. Of course, let it be something reasonable to your lifestyle/schedule. It really will help.

Please don't hate yourself! You didn't do anything wrong, you are just hung up about someone. You will look back on this one day, and you'll be glad you got past it.

If Therapy and Counseling don't work well for you, there are always steps you can take to make the changes you need to be happy: keep a journal, feel good about days that you don't contact the teacher, give yourself the benefit of the doubt, write down what things you have done that you do/don't like. Instead of dreading the future, keep in mind that the future is full of fresh starts - you deserve to be happy, and you'll eventually make enough changes in your life to be happy with yourself.