Why are you beating yourself over the head about Stockxpert's rejection?
It's one thing to want to improve yourself, it's another thing to fall on your kness and kiss the feet of a stock site just to get on it.
Forget them for now and move onto something else.
Keep on shooting for improvements to your work - always, always, always strive for beter,
FOR YOURSELF, not for some stock site. If you do that, then eventually they may accept your images, but you know something? They may not, even if you make the best images in the world.
Don't reduce your self-worth to how many or which stock sites will accept your work.
We are better than that, right?