Well, as another non-native English speaker I can suggest to repeat like a parrot.
I assume your instructor has doubts about ATC may not understand your words and that may cause bigger problems for you.
On the ground: Listen, repeat and try to imitate your voice. Correct your pronunciation. Your vocal paths is set for your native language. So you should practice to switch them to English.
On the air: Listen each word of what you hear on radio. Take notes, don't rush, think what you want to say.