Am I setting my standards too high?

So, I've been dwelling on writing out a decent-sized (probably close to 25-30 chapters) fic. It's my first time writing for the fandom, but I'm very familiar with my source material. Moreover, I've been thinking out some of the details for nearly a year, so I've had plenty of time to weed out most of the ideas that go against canon or a semi-realistic scenario.

Unfortunately, I keep second- and third-guessing my work. It's so bad that I'll freeze up in the middle of writing my first drafts.

- I had a (admittedly sketchy) timeline, but I pitched it when I thought I'd been jossed. I'm still kind of toeing the line there, but nothing's been totally disproven yet.

- The character the story focuses on is known for being difficult to get right. I've seen a couple writers that have him down-pat, but I've also seen a few that are way off. Put simply, I'm nervous about making sure he sounds like himself.

- I'm writing at least one OC, the most important one playing the role of adopted mother and mentor. The others, while not absolutely necessary, are largely there to teach the main character about the importance of family add some flavour to the story. I know that too many useless characters in the background detract from a story, but at the same time, I can't help but feel that sticking two characters in the same house for 15-odd years with a few other canon characters sprinkled in between chapters would be the most boring existence ever.

For whatever combination of these reasons, I get nervous as hell when I put my pencil to my notebook and try to write out my first draft. I'm always worried that someone will look over my shoulder and tug at some overlooked thread that tears the whole story apart. When I type scenes out, I'm always double-checking the flow of my paragraphs. I try to imagine the characters actually saying the lines I write for them; if they don't sound right, they're axed. On top of that, I'm very worried about my OC characterisation - the characters I've created can cross the line into Mary-Sue territory very quickly if I'm not careful about how I use them.

Am I setting my standards for myself too high?

"The first draft of everything is shit."

Assume that is true and just get it down on... paper/notepad/word etc. You can fuss over the details and flowing of paragraphing later. If you're really worried that you'll forget something you want to change later note it down in the margin or... asterisk it or something. My documents are notorious for having 1254973854 notes at the bottom that I forget to delete before posting :B

It seems like you've got a really good grasp on this story, so stop worrying so much and just write it! Writing's supposed to be fun the first time! You can do the boring editing bits later.

I can't help you with getting confidence for actually writing it, but if you're unsure about your characterization, perhaps post snippets or find a dressing room RP somewhere and just bounce bits off people until it sounds right? It helps for me, a bit. But then I don't have the attention span to write long, continuous things.

Now, lissen up 'ere, chilluns; back in granpaw's day we di'int 'ave th' time ta give two spits' worth fer th' stowree's ferst draft. Back then, th' WreeMows wuz in charge a th' joint, an' lemmee tell ya: them varmints could sling a hunerd words a mynoot on a col' winner morn'.

...In more understandable words: don't worry about it (much). The whole point of the first draft is that it establishes the plot/message/general structure of the piece, but isn't necessarily up to snuff in terms of word usage, characterization, or plot holes (or, rather, the desired lack thereof), and its editing is only inevitable.

You seem to have enough of a grasp on what you're trying to write, so you'd might as well get it all down first, then come back to ironing out those little discrepancies only after you're done. If you were to do it in steps, you may end up fixing up errors in one section, just to go right back to them after writing a little more because the information doesn't line up correctly anymore.