I used to be like your DD when I was a child (and as an adult, to a lesser extent). The reason I did not finish projects was because in school, things were easy. But if I did something on my own, I was not used to having to put in the effort and hard work it took to finish a project. And my work never seemed perfect enough for me. I had countless sewing projects, cross stitch pictures and stories gathering dust in my closet. This is an area that I have struggled with even as an adult.
Me, definitely me! I'm still that way. It comes from being a "big picture" thinker but not so hot on the actual details of how something gets done, and the work never fulfilling the dream of the finished project to the perfectionist's satisfaction, and yes, the fact that starting things is fun but finishing them is work that one might not be accustomed to.
Good luck -- if you find a solution, let me know!