You’re right. You need help with this. When thoughts and feelings are this intense, it is difficult for anyone to “just get over it.” I can’t make a diagnosis on the basis of only a letter, but if you were in my office we would do an evaluation for OCD as your concerns are consistent with that diagnosis. If you haven’t already, why don’t you take the online quiz to see if your symptoms match up?
My guess is that your mother, like almost all mothers, doesn’t want to believe that anything is wrong with her child. She’d like to think that you are just suffering from normal teenage angst. Even though it is frustrating to you, it is understandable. Perhaps she would be receptive to a professional evaluation if you calmly suggested to her that it’s a way to settle the issue between you.
I do think that at 18, it’s time for you to stop blaming your mother for not getting you help and to start taking care of yourself. If she continues to be unsupportive, I suggest you first get a thorough physical to make sure there isn’t something medical contributing to your problems. Then talk to your doctor or your school guidance office about how to get a mental health evaluation. If your family doesn’t have good health insurance, ask the same people whether there is a free or low-cost clinic in your area. Then go. You can’t fix a problem until you know for sure what the problem is.
You say you won’t take medicine. But what if medicine is exacly what you need to get back in control of your life? You say you don’t know if a therapist would help. But unless you give it an honest try, you are setting yourself and the therapist up for failure. I encourage you to keep an open mind and to try what the professionals suggest.
You might also find it helpful to do a little reading, both on this site, and in the psychology section of your library and bookstore. Focus on cognitive-behavioral approaches. A smart person like yourself can learn some self-soothing techniques even from a book.
Soon it will be time for you to either go off to college or to get a job and be on your own. It would be great if you could get yourself on the road to a more manageable life before then.
I wish you well.