You're slowly cranking up the contrast, but you need to be more extreme..
It's incredibly easy to get used to low contrast and even raising the contrast slightly might seem like a huge deal, but it's not enough..
You need darker and brighter colors in selective places and you could really brighten up that grass..
Maybe your monitor is has some whacky settings, or you simply need to get your eyes used to higher contrast.
Take the image reference you posted and use the color picker to check those colors..
Try adding some hueshifting as well, like adding more blue to the shadows.
btw, if you want a better software to draw in I'd recommend this:
http://pyxeledit.comYou can create a tileset and then build a mockup and any changes you make to the tileset will be visible in the mockup.