If I'm honest, I can't write for more than 2-3 hours in any given day, and even then, that's if I'm really clear about what I want. I can sometimes sit down, stare at it for an hour, and have only one good line written, delete the rest, and start over later. I'd love to say that I can rip out a script in a quick 8 hour session, but whenever I push myself I come back and read it later and I'm really not happy with it. I did that with the current script I'm writing, and as a result even though I was confident I had about 40% done pretty early on, I ended up rewriting almost all of it. Like, I had the framework, knew what I wanted, which helps a lot, but the details and how we get from point A to point B, the dialogue, these things take time to really feel out.
So don't get discouraged. Take a day or two, play some video games, take a walk, hit the showers, whatever is your zen do that. And then come back when you're ready again. I actually wish more devs would do this, would take their time, because the ones that push through and put out half hearted scripts aren't the ones that make an amazing product. Take your time, be patient with yourself, and keep your audience informed on your progress. They're much more forgiving of time when they're getting regular updates.