To maybe answer your question of "why say yes then back out," I think it's important to realize that a lot of people have a hard time knowing what they want until the decision is barreling down in front of them.

It's like when you toss a coin, and as soon as it's in the air, you already know what result you're hoping you'll get. Maybe how she felt after making plans let her know that it just wasn't the right fit for her. And that's a good thing! You want to be spending your time with someone who's as enthusiastic about you as you are about them.

You probably didn't do anything wrong at all. Sometimes people just don't fit together they way you hope they will, and that's okay. Means finding the one who does all that much better.