Hey, don't give up on that dream. You can write a book. You're only a want-a-writer if you fall into the trap of believing that. And there's nothing wrong with wanting something.
I often find that if I'm not making any progress in a given area I'm practicing, a change in process helps. There are at least two components to skill: practice and strategy. Strategy is what you choose to practice, and skill can't be had without both. If you find your practice ineffective, what's probably needed is a change in strategy.
If you want to write a book, then perhaps a revision of "don't break the chain" is in order: If your writing wasn't coherent, practice making it coherent. One idea would be to devote each week to writing a different chapter of your book. That way, you're still writing every day, but the focus changes each week, so that if you're unhappy with the current week's output, you have a higher chance of feeling better about next week. I realize it's hard to write a book in a nonlinear fashion (earlier chapters guide later chapters, almost by definition, so it's tempting to try to perfect each chapter before moving onto the next) but the goal is to practice writing a book, not to write a good book on your first try. Forcing yourself to hammer out a chapter per week will bring coherence, if not quality. But quality is simply a matter of practice, so keep at it!