Don’t start at the start.
I wrote a lot of my work in sequence, but there was many scenes from Destroyer of Light that were written in segments before I finished Receiver of Many and before the rest of my writing caught up to sequence.
I bet you have a scene burning up your consciousness that’s somewhere in the middle of the book. Even if it’s only a few lines. Or a single idea.
My advice would be to write that scene, then build the rest around it. It doesn’t matter how fast or how slow you write the rest, but it’s going to be a lot easier once you deal with the one thing that propelled you to undertake writing your book in the first place.