Welcome to the forum.
The simple answer is practise! As the letter said, you need to be near-native or strong C1 level. Did you think you were at that level already? For example, look at my corrections in your post above. In the first sentence, you should have used the simple past, not the present perfect. Do you know why?
How often do you use English in your everyday life? Do you speak English regularly with native speakers? Do you study at home? Do you watch TV and movies in English?
It is difficult for non-native speakers to obtain places on CELTA courses because of the necessity for their English to be of a very high standard. There are hundreds of applications for each course and if your English isn't of a standard to compete with native speakers or other non-natives who speak English at near-native level, they won't give you a place.