As much as I would like to help you, and as much as I understand the good will being directed here, I can only say that I'm not a narrative writer. All I do in my spare time is some dumb novels and blogposts solely written for me. Game and script narrative is a fully dedicated profession with a very different approach, not just in the writing and construction, but also in the ways it is edited. I cannot comment on this. It is far beyond my scope and ability.
My only advice to you is the same advice you will hear everywhere else: just do it. Truthfully, sincerely, there's no other way to get better at writing. I can't give you a series of exercises to repeat over and over again, because unlike gym where we are building a specific set of gains, writing is fully dependent on your own personal taste. When I edit other people's work, I am fully conscious of their own writing style, their taste in pacing, their desire for hot steamy fuckery or cold insidious calculations (that may lead to hot steamy fuckery). I do not change any of those. Their personality has to shine through; their thoughts, their observations must be uniquely their own. If it doesn't, I tell them that you have nothing to say. Come back to me when you do.
You need to write and write until you find your own voice. It will be tempting to ape someone else's style. Sure, some people do that. I have heard of those who type out entire chapters to copy a specific voice. I will not begrudge you based on what you choose. But more than anything, I am just asking you to shove those words down onto the page. You will notice, soon enough, if you are a clean first drafter or if you're a vomit and edit tomorrow type of person. I am both, depending on my mood or scene.
The point is to find out who you are. Your ideal audience should be you.
I would also kindly dispel the ideal of a 'dream game' for it places a massive burden on yourself. When starting out, our taste is always all the way up there but our skill unfortunately lies at a different level. Take your time and be satisfied with your output. Your 'masterpiece' will come and go. You'll start work on it only to properly test it as vertical slice and find it trips on the first step. That is not failure. That is simply how it is. How you choose to go forward--to iterate or abandon--is up to you.
There will be another 'masterpiece' waiting. Ideas are cheap, after all.
I was not the original recipient of the ask but I have a couple of specifics to tack on to this honestly already excellent advice, expanding on things that maddie already said above (and throwing maybe a thought or two more into the ring).
First: framing. OP, I get the humor and I get that you're being honest, but be careful with the self-deprecation. There's a fine line between admitting that you're worried your reach will exceed your grasp and that you've chosen a task you haven't the skills for, and reinforcing "this is a mistake I am a bad writer" over and over again. While it's not possible in any world I know to magically make yourself a good writer through the power of affirmation and repetition alone, the more you repeat to yourself that you are a bad writer, the more you will be a bad writer. Not because you never develop skills, but because you foster dead angles and bad habits and other ways that you'll never see the improvement in your writing, because you're busily reinforcing that you are a bad writer and all this reliance on good writing is a mistake.
Second: done is done. Everyone will (rightly) tell you "just do it", but what they don't always get around to is: when you're writing a game, until you've developed your own process that gives you better results, write to finish. Don't write to perfection or to polish: write until each scene has the whole shape it needs, and then be done, and go on to the next scene. Do not be afraid to be messy. Do not be afraid to "write" a line of "dialog" that literally reads "character says catchphrase which will foreshadow the duel on the clocktower in act 4 scene 3 fill in later". Your first gamescript pass is you setting down "OK, what's the shape of what needs to be said". Once you have that shape, you can then refine it, because it's done and you don't have to worry about remembering your references or pre-planning your verbal habits for different characters: you can fill in and elaborate on the script in future passes.
Third: "Masterpiece" used to mean something different. Yeah, these days it's taken as various forms of "crowning achievement" and "perfect representative" but the term was coined back in craft-guild days to reference "good enough" works. A "masterpiece" was alternately "a piece with craftsmanship showing the skill of a master" or, more frequently, "the piece created for submission to the guild in order to prove that a journeyman should be considered for promotion to master". It's a big deal, but it's not supposed to be some lifetime pinnacle achievement: a masterpiece doesn't mark the END of a journey, but rather the START of a journey, a frame change and demonstration of skill.
I'm not saying you'll churn out masterpieces by the dozen if only you start writing, but I am saying that setting your sights on an appropriate definition of masterpiece may well be much easier than you thought; but also be patient, be diligent, and set your sights on the actual, meaningful, actionable "this is a masterpiece; it's my bid to prove I am a master", rather than the busted aspirational older-media "this is my crowning legacy, it's in a class of its own, influencing generations to come".
Following up on Mara's point on 'masterpieces', when I learnt writing, it certainly was used in the way she wrote it out. You were meant to work on your 'masterpiece' every single time--i.e. while it was not meant to be the crown jewel, it certainly was meant to be the best thing you can conceive in the moment because you were pushing something very specific. And working on it will humble you. It teaches you, truly, that ideas are cheap. You never needed to build yourself up to hit that dream. I will write my 'masterpiece' and find that while the initial premise did hit, the rest of the execution remained terribly generic. This too is the learning process.
Every piece you work on should--eventually, ideally--be a 'masterpiece' in its own way. It should test and strain your own limits. In the beginning, everything will. Frustration will come, but understand that this is a good thing.