What do I do about others who don´t understand that i´m honestly sick?

Your Business. Not knowing what your disease is makes it difficult to answer this question, but remember that no one is entitled to know about your disease. You must determine what you are comfortable disclosing and that may be different for different people in your life.
Understanding. In a calm non-defensive manner tell those who do not believe you that they are hurting you by not believing. Then detail the facts of your illness and ask for them to listen and try to understand. I am sorry you are ill, I wish you a speedy recovery.