There are many, many people out there who discuss the differences between art and design. I'll only talk about the difference that is applicable to you, the designer.
Art is perfect. Design is imperfect. Let me explain...
Art only needs to be perfect to one person: the artist. Its "imperfections" in other people's eyes are not relevant to its real perfection. If the artist can accept and love her own mistakes as part of the work, then the work becomes perfect. The work becomes art. Therefore, art can exist perfectly without an audience.
This is not true of design. Design is always imperfect, because it can't possibly satisfy all those who use it. Design's value is entirely dependent on how it is perceived by its audience. If someone loves the design, then it is a good design. If someone hates the same design, then it is a bad design. One can argue a terribly received design as a piece of art—that's also acceptable and fine—but the truth remains that design's imperfection is its weakness but also the source of its strength.
Because design is always imperfect, it can always be improved. Therefore, designers will always have a challenge to do better, even if perfection is an impossible goal.
Why do I bring this up? Because I see many young designers who equate good design with "perfection". And this mentality permeates every aspect of their process, causing them stress and sucking their creative souls dry. These designers want every sketch to be perfectly drawn, every PowerPoint presentation to be perfectly laid out, and every concept to be perfectly rendered. But even a perfect PowerPoint presentation won't make a design great.
Designers: when your focus is on a perfection that doesn't exist, you forget to go deep—deep into the heart of what matters; deep into the emotions of your users; and deep into your own motivations for designing. And depth is much more challenging—and much more worthwhile—than perfection. Rid yourselves of the unnecessary idea that design can somehow be perfect. There's no such thing. Let go of perfection. Just design.