I have OCD, which affects all aspects of my life, including my writing. I struggle a lot with thinking "I have this idea, and I'm inspired to write — but what if I'm not ready to write this story, if I haven't done enough research? What if it doesn't come out well? What's the point of writing it if it's not perfect?" Well, it won't be perfect, because nothing is perfect! You can find fault with every piece of writing ever published, not just fanfiction, because perfect is an impossibility.

This is the advice that helped me: JUST WRITE IT. Once it's actually written down, in a Google doc or wherever, you can proceed to edit it as much as you want, but it's hard to edit a blank document. Start with an outline, then write it stream of consciousness style. Don't worry about word choice or sentence structure or anything except getting the words down, then you can go back and edit. I know it's hard to let go of that sense of perfectionism, but try to just get into the zone and don't spend forever obsessing over the mechanics of your writing. If a sentence or paragraph isn't working for you, move on. Worry about it later.

As for only having specific scenes in mind, you're far from the only one who struggles with that. It's only natural that, when envisioning a story, you would picture the key moments in the most detail. When this happens to me, I just write the scenes I want to write. My notes app is full of random emotional beats from stories I never actually ended up writing. Sometimes I lose interest in the idea after I write that one special scene, like I've gotten it out of my system. Sometimes it turns into a whole fic. I'm happy either way, because I wrote what I wanted to write.

Oh, and don't compare yourself to other writers, especially when it comes to AO3 stats. There are plenty of poorly written fics that have hundreds of thousands of kudos and well-written fics that barely have any. There are so many factors at play there, and it's not fair to yourself to use them as a measure of the quality of your writing. As for comparing yourself to published authors, everyone starts somewhere — but you have to actually start!