Many students do a lot of testing but not enough preparation. The problem with tests is that you don't learn anything new. You don't improve.
"To train for a marathon, you don't run a full marathon every day!"
The solution is to spend more time preparing. Don't test yourself on a new topic that you have no idea about. Spend some time researching the topic to find good ideas and vocabulary. Then try to write a 'perfect' essay using your research, a dictionary, your teacher to help you etc. Type the essay on a computer, check for spelling and grammar mistakes, re-read it, look at how the paragraphs are organised, highlight the 'band 7 vocabulary'. If I've covered the topic on this website, use my ideas to make 'perfect' paragraphs and ask a teacher to check them.
When you have written a 'perfect' essay, wait for a few days and then test yourself on that topic. Imagine the difference!