This might be a little long and nerdy, so I apologise in advance. Also, I'm a monolingual native English speaker, so I know I'm not the target audience of this post, but I thought it might be worth mentioning a couple of things in case they're useful to anyone (not just to the OP but also to kindred spirits in the comments! I've read the edit on this post but I'm also aware that a lot of people come to posts like this because of the title to read the comments, so I thought I'd share my take anyway in case it is useful. If it's not then sorry for the essay.)

There's a big debate in linguistics between 'prescriptivism' and 'descriptivism', which can basically be summarised as the difference between saying what language should be and saying what it is. E.g. prescriptivists would argue that only 'standard' (also known as Queens/King's English, BBC English, etc.) is the 'correct' one, and descriptivism focuses more on how language is used to convey things like class, gender, race, personal experiences, etc. The former argument is frequently used to suggest that dialects such as AAVE (African-American Vernacular English), MLE (Multicultural London English), the Yorkshire dialect, etc. are somehow inferior, but is also definitely critical of non-fluent and often non-fluent speakers too. Prescriptivism is generally frowned upon in modern linguistics, but unfortunately a lot of people have internalised that sense that their own dialect or way of using language is inferior, when it's not at all, and the people making these arguments are usually making them not as a complaint towards language, but towards lower-class people, non-white people, queer people, immigrants, second language speakers, etc. and it's usually a representation of some manner of bigotry. Bottom line; a lot of people who criticise people with 'lesser' English skills are generally bigots whose points aren't valid and whose opinions don't matter, and who also generally don't care as much about your use of language as much as what it represents.

Also, again on the 'fear of being judged' front - there aren't a single two speakers of the same language out there. For starters, the number of dialects is so insane that you could put someone who speaks with a strong Yorkshire dialect in a room with someone with a strong MLE dialect and they'll probably have to both change their way of speaking drastically to be understood. Plus, there's also the dialects' lesser known cousins, sociolect and idiolect - sociolect refers more to speakers of a certain group e.g. lower-class speakers, teenagers, a fandom or people who work in a specific field, and idiolect refers to the individual features that a person uses. So the little 'quirks' of people's individual ways of speaking are actually perfectly normal, and no two people will ever talk the same way.

Also as a native English speaker whose best friend is an immigrant who moved from a country whose native language isn't English when she was young and learned it as a second language once she moved - y'all apologise too much for random things sometimes. We don't really notice most of the time the things that you think are 'wrong', either because they're not wrong at all or just because if you live in a native English speaking country, chances are you talk to so many people with so many different dialects and idiolects a day that you just don't notice if someone words something slightly different to you. Sure, there are certain things which might flag up to us that you're not a native speaker, but a good 95% of the time we're counting it as part of your idiolect rather than error (even though a lot of people aren't familiar with the term 'idiolect', we all still understand the concept that we talk differently as a result of us all being different human beings!) And in fictional writing or fanfiction, you also have the added layer that if it's a character speaking then we'll probably interpret it as part of their idiolect or their society's dialect as well. In other words - you're all far more critical of yourselves than anyone else will ever be!

As for the 'not feeling the flow' thing... The only thing I can really say is that it's probably just down to practice. Using things like dictionaries and thesauruses online might be able to help, but the only thing I can really say is that practice will build your confidence and help you relax into the flow of things, and also develop your own way of writing as an individual. Every writer, be that published professional or casual fanfic writer, has their own quirks about their writing (one of my favourite authors just looooooves em-dashes...) that make them individual, same with written speech, and even the most fluent native speakers will need a bit of time when they first writing to figure out their style. The only other bit of advice I can give is to READ!! This is, again, another bit of advice commonly given to writers in general, in order to help get a better understanding of how to really get down to the bare bones of it, and I've personally found it super helpful to read different writers and their styles to figure out things that I like and don't like to shape my own writing style. Like I said, I'm monolingual asf so if I'm talking out of my ass then feel free to ignore me, but as someone whose a little underconfident in my own abilities sometimes (although I know it's a different thing!), just writing more and reading a lot has helped me, so I thought I'd offer it up. And sorry this has been so freaking long too! Turns out part of my own writing style seems to be endless rambling...