Is your daughter bothered about the fact that the projects aren't finished? Maybe you can redefine the types of projects you do so they feel more open-ended and can be put down at any time without a sense of failure or incompleteness?
For example, my dd felt bad about the fact that she got bored with her knitting before it was finished. Now instead of knitting a hat, she just starts knitting a long thin thing, LOL. When she's had enough, she looks at what she's got. It might be a dolly scarf, a teddy pinic blanket, or a Barbie blanket depending how big it has grown.
I don't think all children feel the need for completion in their learning. Why slog on once her interests have shifted? The main thing is that she should enjoy what she's learning and leave it hungry for more. If she feels she must finish whatever she starts, she may become reluctant to start. She can always come back in later years to look at things again if she wants to. What I know is not very systematic either. I know more about the Victorians than the Edwardians, more about Spain than Italy, more about the European exploration of the Americas than the East Indies. I only know a few languages reasonably well, though I can remember having started another eight or so. Those bits and pieces of other languages are still valuable to me and have helped me bumble my way through Belgium and Portugal.
Things become complicated in later years if there are formal courses involved. I've forgotten how old your daughter is; is she anywhere near wanting to do college-level study where a lot of chopping and changing might be frowned on? If not, perhaps it's better to focus on what she has done rather than on what she hasn't. Think of the projects she has begun as so many sparks that have been lit.