As a composer (of music) I've come across this with every single piece I've composed, and I'm assuming it comes with the territory of being in any creative field. What happens is you "live with" this thing (a piece of music, a novel, a product) for far, far longer than most of your audience, and lose perspective. It starts to seem like crap to you only because you know all of its tricks and they are not amusing to you anymore (like watching a magician all day for weeks on end).
While I can't tell you specifically what it's called, I can give you some advice which may or may not be practical: work on more than one thing at a time and move on to something else when you start feeling this way. It helps to put some space between (for me, one to three weeks). When you come back to it after working on something else you will remember a little bit of what it's like to have that initial spark that prompted you to start the project in the first place, the same spark that your users will feel whenever they use it after moving from product to product (piece to piece, film to film, etc.).
However, even this strategy wears off after a while, but hopefully long after the product has been released.
And the other side of this is Caleb's answer. You might not recognize just how unintuitive something can seem to someone who hasnt lived with this thing like you have. The above advice works for Both cases, though. And if you can't afford to take that kind of a break, then you have to bring outsiders in to critique your work. That's exactly what novelists, playwrights, and others do in situations like this.
