Stitching Multiple Images Together

I took some advice on making multiple images of the same scene with different shadow depths and stitching them together for a more realistic appearance. Unfortunately, the most recent image I made has different light levels, even though I didn’t change anything other than the shadow depth between shots.
Below is a link to the image as of right now. You can see around the more dispersed shadows in the background that the lighting/coloration differs from the rest of the shot. Anybody know a good way to fix this lighting mistake? I’m fine with any methods, from ingame to photomanipulation.

Imgur

PLEASE NOTE: This is not one solid image. I made two separate images and pasted parts of one onto the other, causing the uneven light distribution. (Sadly, I need to point this out)