Unfortunately, your family is experiencing a difficult challenge. Understandably, your parents have a heavy load. They are struggling to help your brother and as a result, though not intentionally, they are not meeting your needs.
You mention that there are many people who are experiencing things that are worse than your situation and that may be the main reason why you feel unworthy of help. Many people experiencing your predicament would feel the same way but anyone in pain deserves help. You needs are not being met because the family situation is difficult. Everyone is being negatively affected.
Please know that you are deserving of help. You have the right and the obligation to ask for help. If your parents cannot see that you need help, then try to make them aware. I understand your reluctance to speak to the guidance counselor but it would be a step in the right direction. Inform him or her about your situation. Express your concerns and perhaps he or she could convince your parents that you need help.
Another option is to discuss your concerns with a trusted family member or family friend who may be willing to speak to your parents for you. You could also show your parents your letter to me and my response. The fact that you took the time to write to a mental health professional may help your parents realize just how much this situation is affecting you. I wish you well. Please take care.