Rebecca,

As much as it is true that your thoughts are yours alone, you do not have to carry the burden of anxiety all by yourself. Long term anxiety is not caused by you, its an imbalance in your body. You are not going mad, millions of young adults, like myself, struggle with anxiety everyday. As much as you need to accept this, you should also know that you are the only one standing in the way of feeling much better. I am in the process of dealing with anxiety now, and I would also feel as if I was on the edge if I was trying to conquer it on my own.
The best way to overcome your anxiety is to get help from others. Therapists do work. And if they “stop working” then you are simply not seeing the right person. Depending on your individual case, some herbs or medications might help. Please don’t dismiss this. If your anxiety is affecting your life, like taking a toll on your academics as you mentioned, treatment, let it be just therapy and/or medication, is really needed. Being so close to your age, 18, getting your emotional health in check before you head off to college is a must. There is a good chance I would have dropped out by now if it wasn’t for the treatment and support system I have developed.
Please take all of what I said to heart. I have been in your shoes and I know how difficult life can be as a young adult with anxiety. If you don’t know where to begin in looking for treatment, there are services at your school (both high schools and colleges) that can help point you in the right direction. Most everyone you reach out to for help will want to try their hardest to get you on the right track; all you have to do is let them know.

Sincerely,
Celeste