The third novel in my series has been so far the most difficult yet. (I'm about 32k words in, expecting it to go 150k.) I know more about what I want to happen in this than I did my first two, and yet I'm finding it hard to find the way I want to tell it.
I keep reminding myself of advice I saw from Neil Gaiman about being a writer - an article with lists of rules from various well known authors, it was Gaiman's that stood out because he said it all so simply. I don't have the link to hand but the first rules in his list were all effectively the same: Write! A later rule said: Finish what you start.
That advice is what got me through the first books, and what I'm hoping will get me through the next ones. Nothing happens for me unless I write. The story goes nowhere until I get in there with my characters. It could be just me, but that's why Gaiman's first rules struck a chord for me.
Also, I tend to be schizophrenic about my writing. One day I won't like what I've done, the next day I will. This was especially bad with my first book. And yet, when I finally got the end and read back over it I thought: Hey, that works. It wasn't Shakespeare, but it was what I wanted it to be. And once I had the completed story I had something I could really work on and polish and adjust. And that's why Gaiman's later rule still rings in my head, I would never discovered if the story worked if I hadn't finished it.
Not that I've actually finished everything I've started, but since reading that advice I try to give everything at least a good chance. Some things just refuse to move.