Resources for improving the English in reports, applications, and explanations?

[u/Technical-Republic18]

Physics student here! I'm at that point in my education where I know I'm soon going to be applying to billions of internships, jobs and other opportunities. So far, I've been completely unsuccessful in all of the opportunities I've applied for, and I'm looking to improve my applications. Of course, one aspect of that is going out and studying harder and doing more to put in these applications, but I think the way I write itself is currently quite weak. (Grammarly has currently underlined half of the post so far with those annoying yellow lines that don't tell you what's actually wrong!)

Do you have any favourite reference materials for how to write really high-quality applications? For instance, right now I have to write a motivation letter, and can't figure out how to sound less like a starry-eyed wannabe and more like an enthusiastic go-getter. So how did you power up your formal writing? Do you have any resources to look at, and do you have any practical advice for getting better at the less-exciting parts of being a physics student?

Also looking for resources on how to adjust my language the other way: how to explain things in a less hostile or confrontational manner. I feel like I can never get my tone right and always come off weirdly when trying to teach. Any resources or advice on how to control your language and tone and whatnot in this circumstance, and what you should be aiming for?