Sometimes, you just have to push through the initial dread. Think of writing a new chapter like going to a new place with new people. If you're shy like me, you'll know the feeling, but in a new place with new people, I don't want to do anything and melt back into some corner. It sometimes feels easier and safer to just be alone and not interact. However, you probably also had those times where you have interacted and it turned out to be one of your best times of your life.
If you can't relate to it, then think of writing as a drug.(a healthy one and I'm not promoting drugs in anyway) Your first high feels great, and it probably didn't take a lot. Maybe a 60-150mg dose was good enough. Now, as you build up tolerance to the dosage, you need more and more to achieve that high. However, you can get the same effect by doing a detox for however long it takes and let your tolerance level naturally go down, then getting high again.
Writing can be the same way as both of these examples. If you want to stay motivated to write, either break through your unmotivated mindeset, or take a break and let your tolerance, to the dopamine from writing, go down.