Moira, the answer to your question really depends on how a person is 'wired'. Some people like to make one thing at a time and see it completed before they go on to another. Others of us like the creative process and flit from one project to another like moths to flames. Then there are many of us that lie somewhere in between.
For me, if I have started something and lose interest, I don't want to feel enslaved to getting it done if I am no longer interested. I think we all believe we will eventually "come back" to those projects and get them done... hence, UFOs.
Sometimes I feel undisciplined when I start something new but still have several unfinished. But I try not to beat myself up over it. This is supposed to be fun and I'm in charge of what I work on and what I finish.
I admire your discipline!