1. Going back and forth between tweaking and rewriting
Especially in the early stages of narrative a writer will be temped to start editing which will increase the pressure and will make it hard for any writer to get past the opening chapters. No matter how much you may feel tempted to revise while the book's not finished, stay away from it because it's too early to decide whether the pacing is right, a character has enough prominence etc.
I don't know about you but I keep rereading the first chapters of a new novel. Even though I don't necessarily tweak and edit at this point, rereading is just as bad because it clouds one's judgement due to the work seeming over-familiar and makes one focus on the flaws rather than the good points. That can prove just as discouraging.
3. Being over-critical
Rereading is necessary, particularly to remind the reader where they left off. In such a case, it's important to avoid being over-critical due to outside influences such as having had a bad day or lack of work motivation. Don't become jaded just because you're tired and feeling low; only reread when you're enthusiastic and feel positive.
4. Don't show your work to lots of people
This is probably my favourite tip because it's one mistake many writers make. Getting feedback might seem like a good idea, but receiving conflicting advice and opinions will only frustrate you and make the book idea less appealing to you. At this point in the writing process you need to keep a clear head, a definite vision and a high enthusiasm.
To your publishing success!