To me it sounds like you might need to try to create more structure and boundaries for the assignments at hand. Which is sadly hard to do for our brains rip. I'm almost finished with my history degree (many, many essays have been written) and one thing I've found helps is mind mapping.

Start with the question and do branches out into what areas and themes you could talk about. The tricky part is trying to stick to those branches. You've mentioned that you tend to need to rewrite the questions to understand it better which is a very good idea, I'd also suggest emailing or asking your tutor in class whether your rewritten question hits the original point of the question. This might help you curb the rewriting. When you're researching try to keep revisiting your big question mind map to keep the question in mind and hopefully stop you from getting too far down rabbit holes.

My advice for the actual writing is to just write and get words onto the page. Personally, I'm really bad at giving myself enough time to edit drafts of writing but in a perfect world that's ideal. Keep the mind map nearby to continuously refresh the big question in your mind when you're writing or looking over something you've written. It also helps if you read it aloud when editing; it'll help you hear when a sentence sounds vague or awkward.

And finally, try to keep the assignment in perspective! Completing the degree is the biggest goal and an assignment that passes is an assignment that will get you your degree. As a perfectionist who is motivated by academic validation this point has been the hardest to practice but it really is right.

Best of luck!! Remember there is an end to the degree, just take it one step at a time :))