I'm sorry you are having these difficulties.
It is very unlikely that your teacher would deliberately give you a worse grade than he thinks you deserve, just to be mean. Teachers are professionals, and they have been trained to be as objective as they can.
Unfortunately, it might be true that the teacher does not think you deserve top grades. Impressions are relevant, and if this teacher believes that the smartest people are the ones who answer in class, then it will be more difficult for you to convince him that you (a quieter student) have really grasped the mathematics. You need to prove it to him by always producing perfect homework.
Would it be possible for you to pay for an outside tutor in maths? If you could find someone that you are comfortable with, it might help you to learn.
If a tutor is not an option, consider online maths courses. Have you tried youtube videos? There are some great ones out there. Whatever you choose, the way to put your mind at rest is to put a lot more time into the maths work, so that you know you can ace every question and you deserve an A*.
Looking to the future, are you sure you want a career that relies on a top maths grade? There are many ways to live a good life, and a university course is not essential for all of them.