Design is temporal. It lives at a certain moment in time and is evaluated against the backdrop of culture. As culture continuously evolves, so does the best solution to any single design problem. Perfection is contextual: The best creative solution to a problem is only ever the best one at that time and place.
What about those "timeless" designs? What about those objects from 50 years ago that we still covet (I'm looking at you Dieter Rams). What about the pieces that everyone agrees are at least as beautiful as when they were created—if not as functional—and have not since been improved upon in a significant way? Truly great designs are longer lived, almost eternal. That is because "timeless" designs get at visceral, fundamentally human responses, and thus are less affected by ever-changing culture. These emotional designs tug at deep, biological preferences rooted in our animal nature. Unfortunately, the only way to really be sure a design does this is to reexamine it later on.
That's why design is such a moving target. As a designer you are trying to meet your own impossibly high expectations. You are battling design constraints while balancing client demands and your own standards of quality. Your only recourse is to fall back to your design process and rely on your battle-tested tools. Maybe you work directly out of The Universal Traveler, or maybe you need to shuttle back and forth between brainstorms, mind maps, and Post-It ridden whiteboards sessions. This is something many young designers entering the field struggle with. They have not been consistently using their process long enough to place their full trust in it. I am familiar with that feeling myself. It's a nagging anxiety about the final product, the fear that what I create will not measure up. Fear is the natural enemy of creativity, and it leads to half hearted design decisions and design directions not fully explored.
So take heart, designers! Tune and follow your process diligently. You only have to worry about making the best design possible for here and now. "What looks ripe today can become moldy tomorrow" (Don Koberg and Jim Bagnall, The Universal Traveler).