I can totally relate. This is a part of ADHD too and I've always thought of it more in terms of ADHD, but I can see how it ties in with Bipolar as well.

I get bored very easily and, like you say, I don't tend to stick with things. But then, when I get something that really grabs my attention, I go the other way and become completely obsessed with it and spend every waking moment doing it. That's definitely another ADHD trait. And it's not healthy either. Its very hard for me to do things in moderation.

ADHD meds can be very helpful with this aspect of ADHD (and other aspects too, obviously). Unfortunately I can no longer take them because it turns out I have a heart arrhythmia and have to stay away from stimulant drugs. Pretty sure stimulant drugs wouldn't be good for people with Bipolar either....

Sorry I don't have any answers! Just wanted to say I can totally relate!