You did a good thing by having those diagonal lines in your painting. These are called oblique lines and in paintings they are the most appealing lines before vertical and horizontal lines. So there is nice visual movement. You managed to create a good sense of distance in your painting due to the following reasons:

• You have three planes: Foreground, middle ground, and background.
• You have atmospheric perspective by painting that background mountain in a blue hue.
• Having darkened the immediate background was the right thing to do. This helps create the sense of illusion because we have a progressive value change.

Another good point; there is good color variation in the greens on the grass. The orange hue in the grass makes it more appealing and provides relief from the monotony of the greens.

Shaping Your Trees

Trees should be studied in detail. They have "anatomy" of their own. Try to avoid painting them where they would almost fit in a perfect circle. I cropped the top of the tree so the painting would be "anchored" to the
frame. Also by eliminating a portion of the tree, I took away all the attention it was receiving. I also added sky holes in between the foliage to give it an airy look. This avoids trees to look like they are cut out and
pasted on giving them a sense of belonging. This in my opinion conveys the idea that the leaves are blowing in the wind. Also I darken the areas where the direct light doesn't hit to give it a more round feeling. I added a stronger tree trunk to give it strength.

The Center of Interest

The barn definitely became my center of interest (usually located at 1/3 of the frame horizontally and 1/3 vertically) As soon as you add a man made object to a landscape it becomes the center of interest, I felt that the original painting was lacking a center of interest.

Adding Another "Plane"

I added one more plane (pine trees behind the barn). This helped create an area of strong value shift from light to dark, thus making the barn pop out. This also avoided the feeling that the middle ground ended too abruptly. This way we have a smooth transition from the grass to the trees and then the mountain.

Conveying Distance

Using your idea of darkening the foreground to convey distance, I went ahead and even went one value darker.

Additional comments:
Congratulations! Well done! I would recommend that you visit the ArtSchool Online section of WetCanvas! There are many good suggestions for painting techniques. You may want to paint this again using my ideas and see what it looks like. I'd like to see it again if you get around to doing it.