Someone once told me that there are two kinds of dissertations.
The first is done really quickly and is not very good.
The second takes many, many years and is also not very good.
Of course, this is tongue-in-cheek. But the point is to finish. A dissertation is not meant to be your magnum opus. You're trying to please your director and a committee, and you have to keep your topic under control, no matter how interesting the rabbit trails. It's good to remember that a Ph.D. is only one step in a lifetime of learning. Finish it, revise it for publication if it's at all feasible, and keep on researching and writing... but hopefully then you will have the option of getting paid for your work.
In the hopes that I have less than two years ahead of me, I keep reminding myself that the most important thing is not impressing the whole world (or even just my department), but rather finishing the dissertation.