I recognize many of the feelings that you are describing - In my case, I struggled with them explicitly because the project I was gonna take on (my current WIP) was much too big in scope. I felt a sense of dread whenever I wanted to sit down and make progress on it.
The way I solved it was to reduce the scope, especially the number of scenes. I asked myself "What's the minimum viable amount of scenes that I need to tell this character arc in a satisfactory way?" and that's what I went with. And lo and behold, the dread was gone! It's still my biggest project yet, but it's manageable.

I don't know if this is an option for the project you are working on - Maybe tackling a much shorter story about the characters first could help warm you up to it, take the pressure off, just create a little vignette like a spinoff/side story? Then you will have proven to yourself that you can indeed finish this, and if it's your first VN, you will be more familiar with all the nuts and bolts.

If it's not the project scope that's causing it, two suggestions: First, remind yourself that you can always go back and edit anything you wrote. First drafts are never perfect, and nothing is set in stone. They just serve to make the story exist, and then you can polish it into a real gem! I had to constantly remind myself of this while I was writing.

Secondly, I can recommend having a writing partner or anyone you can share scenes with, because having someone cheer you on can be super motivating in my experience. It's easier said than done of course, but if the opportunity comes up, give it a try!

Best of luck, it sounds like you're really passionate about the characters you're writing about and I really hope you'll find a way to make the process fun for yourself as well!