How can I improve my grammar and sentence structure.

Hi! for some context I am a tenth grade student in french immersion in Alberta Canada. French is a massive part of who I am. Be four my grandma passed, she requested I graduate in french immersion as it was her first language. I am an honours student , I have 90s in every single class except french. My teacher tells me its my grammar and sentence structure that is taking so many marks off. Now id like to say I got a 65 percent on my last essay in french class. For another perspective in english class i've gotten 100% on every single essays i've wrote. I was wondering what I can do to improve my grammar and everything that goes with it. This may seem small but the stress is taking a big toll on my mental health. The reason being is my future carrier requires me to have high 90s and french is dragging it down. I also have my b1 in delf which I unfortunately got 11 out of 25 on the writing. Any suggestions would be appreciated.