One of the hard parts about writing for me is starting again without comparing what I'm doing today to what I wrote yesterday.
The part I've already written always sounds pretty good (at least, before editing) and the part I'm trying to work on doesn't seem like it's even worth writing down. Maybe this is self-doubt creeping in, or some kind of writing perfectionism. Do you have this problem? Some ways to deal with it:
1. Don't go back and read over too much of what's already there. It's too intimidating. I just go back a paragraph or so.
2. Leave a quick note at the end of each day's work about where the story is going so I don't have to read the "better" part of the book over again.
3. Begin writing in a fresh document and copy it into the rest of the novel later (if it's good enough).
I know some writers don't write sequentially, so maybe that helps.
When I start a writing session, I always think my writing should flow seamlessly on to the next section. What I forget is all the re-writing I did of each previous section while I was writing it.