One of the things that breaks my heart on here is seeing fanfic writers - and I encompass everyone from those posting 75,000 word epics on Ao3 to those posting quick drabbles on their blogs - beat themselves up because they feel their writing, spelling, grammar etc isn't to a professional standard. Or worse, they're sad / hurt / discouraged cos someone left a mean comment criticising the same.
So I dunno if this helps but, take it from someone who has been both a professional writer and editor for decades: the professional work you are comparing yourself to didn't start out like that either.
True, standards in professional publishing have slipped - cursed by the same cost cutting and profits over people mentality you see everywhere. But the chances are everything you read has been through at least 3 or 4 pairs of eyes before you read it: editor, copy editor, sub-editor, proofreader, to name but a few potential steps in the process.
I've edited some incredibly talented (and sometimes very successful) writers and there has literally never been a single instance where I changed nothing. I've given things what I considered insanely close edits only to have proofers pick up on *my* mistakes.
I've written for some very prestigious and some very well-paying outlets - those things alas rarely overlap - and even my cleanest, tightest prose has still generally seen some changes before it ended up in print / online (I've also had some absolute howlers mercifully caught by an editor or proofer before my shame could be published!)
There are some incredibly talented writers in any fandom you want to pick - often churning out volumes of stories that would put a professional to shame. There are also newbies who'll get better with practice and people who just don't have the gift and might never be polished but hey, they want to be part of something and express their creativity and share something they love with people who love the same things. *All* of that is valid and valuable.
So my advice is stop comparing your work - even if it's been beta-ed, even if you've done half a dozen drafts - to work that is out there as a result of a paid, professional team polishing it. You're setting yourself an impossible goal.
And as a reader? If poor prose or the odd grammar error bothers you, stop reading and move on. Don't be a dick about it.
Or accept you'll get more out of work if you meet it where it lives. You don't go to see a pub band expecting Taylor Swift level production values. You don't go see a little Fringe show expecting the West End. That doesn't mean those can't be some of the best experiences you can have.
And if that doesn't inspire you: you're still better than someone using AI to turn out heartless slop!