I feel really sad what should I do?

I am 14 and I have been sick for about 7 months. Even though I been in hospital 8 times, had heaps of tests, an operation etc we still don't know whats wrong with me. Last year I missed over a term of school. I can't go out much because I get really bad stomach pains without warning, sometimes so bad I cant walk. When this all started the then head teacher of my year was mean. She started sending mum mean emails and accused me of pretending. I am doing distance ed this year instead because I can't go to school enough. My doctor finally found a sickness I have but now they don't think that is the main problem so I am still undergoing lots of test. For most of this time I have been felling fairly happy. But just over the last 2 weeks or so I have started felling down and a bit lonely. My friends still visit me but I don't get to see them as much as I used to. I don't want to tell my mum I am felling down, she has plenty of stuff to worry about. I just don't know what to do. Please Help