If you are in your second year of junior high school, you usually get good grades and have high expectations of yourself, but recently when you encounter homework that you can't do, you start to suspect that you are not as good as your classmates. Every time before the test are nervous, worried about the test, afraid of learning to keep up, and even sometimes say that they are useless, this negative mood is really dangerous, need to be adjusted in time.
From your description, you seem to be a bit of a perfectionist. Although you didn't ask me directly, after a few days and seeing that no one had responded, I took it upon myself to share some advice, so I hope you don't mind my presumption.
In your case, I think you can try to lower your standards a bit. Remember, no one is perfect, everyone has bottlenecks and mistakes, there are always things they don't understand. So don't be too hard on yourself and focus on your own progress. If you don't think you can do it, then find ways to improve, such as practicing more or asking your teacher for help, other people's performance has nothing to do with you, focusing on making yourself stronger is the key.