I'm chinese but I can't speak it?

I'm full chinese I go to an immersion school. At my school there are other people that are half Chinese and can speak it fluenty and write to but I'm full I can kind of write and speak it. Alot of people in my class always say why are you full Chinese and you can't speak it. I feel like a failure. I was spoken in Chinese to when in younger and used to speak it all the time but now I can't speak as well. Why is my Chinese so bad I say this again but I feel like a failure and ashamed I did a worksheet that we did in Chinese about grammar and I got them all wrong and others who are not even Chinese got most of them correct what is wrong with me. Serious answers please