Yeah it's a bit hard :S. I also screwed it up by not painting any perspective lines .
I'm not sure if I'm gonna add more details. It was one of those works, where I was actully forcing myself to finish it :S. I would rather start a new one from a different view/perspective then contiune this one .
You're doing very well. I could give you few small tips that would greatly improve your painting .
1. Place the camera lower to the ground 2. Make the perspective more 'aggressive'. Let the objects in the distance become smaller much quicker 3. Use a grid to help you with the perspective. 4. Download free 3D modeling app and create some simple scene, then pain over it. This REALLY helps with creating proper perspective
Here's an example of a simple scene [link]. You can easly add more details and own lighting information while keeping the perspective in place .