When are you fully committed?

It's time for a new idea and I truly don't know what to write. Brainstorming is my favourite part of this writing process, and at this point, everything is on the table. I'm committed to absolutely nothing, not even genre, so I'm really going where my best idea takes me.

I like to plan in advance, have a strong sense of the major plot points, but usually I jump into a book too soon, a mistake I'm hoping not to repeat.

So my usual process is, I brainstorm a general plot, the beginning, the end, the internal and external conflicts and then, when I should be thinking more of developing the relationships between the characters or how the character arc plays out, I get impatient to start and I write the first chapter and go from there. Once the first chapter is written, I'm pretty much committed to the idea and I rarely back off from there.

I know people, who are committed to the idea almost from it's inception, and others who can write three chapters and still back away from an idea.

But by the time I start the first chapter, I'm so excited about the prospect of the story that I get caught up. It's nice, it's a place for me that has little doubt, and that's about the only part of the writing process that lacks doubt.

Anyone gotten farther into a book and turned to another idea?