how can I better understand active voice+repetition of words+sentence flow?

I’ve working on a historical paper for school and I tried testing it on those grammar checking sites and apparently It has around 70 problems. My paper is around 7 pages long with about 1700 words so maybe thats why but I really want to fix it. I can I do this easier without rewriting the whole thing and then going through an identity crisis? Also if you have tips on how to not repeat words 50 times and have a better writing flow, do tell me. If you are an absolutely amazing person and would want to give direct advice to my paper give me a DM and I’ll send you a copy of it!!!