I used to be a 'serial quilter' too but then something in my brain clicked and 'allowed' me to have multiple projects! You say you're worried about the quality of your work if you multi-task - I feel for me that I'm more likely to do second rate work if I MAKE myself finish something I've lost interest in than if I switch to something else. Everything will get done in the end - or not but hey, it's not the end of the world. :mrgreen:
The exception is commissions or gifts with deadlines or pieces for competitions. If there's a deadline, you just HAVE to get it done. The other stuff, well I do what I fancy and when because even though I now am partly doing this as 'work' at heart, it's still a hobby so if I'm not loving it, what's the point? :-D