RedBubble - success?

In my level 5 end of year presentation, I mentioned that I had created a RedBubble account. Initially it was just so I could see what my work looked like on various products - because I was too lazy to photoshop it all myself. Over the summer however, I've actually had some success from it! As of now I've had 54 sales, which to me is crazy considering I never thought I'd get anywhere with it.

At the moment however, I don't really feel like the work I've got on there is much good. Whenever I open up the front page I always feel like everybody else's work is so much better than mine - especially the t-shirt specific designs.

From now, I'm going to focus on making my designs better, so I don't feel so inadequate when I log on and see all the new uploads. What makes a t-shirt design a good one? How do I stop my designs looking like they've just been slapped on a shirt? Am I being TOO critical of myself? There are so many things I want to improve upon this year, I hope I don't drive myself insane.