The brain is a very complex and fragile instrument, much like this watch. There's lots of gears and springs and little parts, and if something is slightly out of adjustment, the whole thing can come to a grinding halt. Consider bipolar disorder like grains of sand in there. Sometimes, they're contained and not doing any damage; they just kind of sit there. Other times, the grains get kicked up and lodge in the gears and bring everything to a stop. This has been the case with a lot of my hobbies and interests over the years. I used to be a trumpet player, and a fairly good one at that. During a depressive phase, I gave up on it. The same goes for drag racing, building model kits, rocketry, playing guitar, and even photography, though I've come back to that. What happens is that I get into something wholeheartedly during a hypomanic phase (one of the symptoms of hypomania is grand ideas and starting major projects), then a few phases later, a major depressive phase hits, and I lose interest in what it was I was doing. That major depressive phase is the sand in the gears. It could stop me despite how determined I was to succeed at something.
On the flip side, the hypomanic phase can also put a wrench in things. The grand idea aspect got me chasing too many new projects to finish any of them. A good example would be with painting projects around the house. The trim in my dining room still needs to be finished. I have to paint the plaster above the stove. But the dining room is painted and the hole in the wall is patched. These were projects abandoned before I could complete them to work on the multitude of other projects and ideas I came up with, all of which are in the same state.