As you upload, you start wondering what are the stock rules about? What is quality for reviewers when they often reject quality for other reasons? Over time, I learned to respect their decisions, and to treat rejections as natural. As soon as I started thinking about a rejection as of a "We've seen you do better than that! Do that good stuff you do again" and not as of a "You suck" phrase, I started enjoying this struggle more. It's all about making an effort, like with everything else in life.
"If you sold one, you can sell more! Don't stop, just try another approach - Someone already appreciated your work and spent money to be able to use your photo!"
When I started uploading to stock photography websites, I was careful while choosing photos. Then I made a mistake and uploaded almost 200 photos (generally beautiful), but without sufficient checks and analysis, to a website which usually approves my work...The result was staggering 90% rejection!!! In that moment I did not consider that it was actually my fault, and I was - of course - dissapointed. I stopped uploading for a month, and then I accidentally read an article which said something like:
-so I started trying again, and it started working again. If you make a strict selection yourself, you avoid situations in which someone will do it for you. Learn to recognize your own mistakes, and it will boost your confidence, especially because you'll definitely give your best not to repeat them.