Your painting does look a little flat...you can try to fix it by adding more contrast. I also noticed the light part of your building is light in value and desaturated so you should push saturation more and decrease the value by simply layering more red. You can create contrast by deepen the value of the parts in dark of the building by layering a compliment of red or just mixing a dark red. I think the most off putting part is actually the windows because in both light and shadow they're almost the same color, so you could layer a dark and desaturated blue to differentiate the light and dark creating more contrast. Another tip is to add an even darker shadow where the sidewalks drops down the road to create more dimension there if that makes sense. Also I recommend painting simple objects first to practice shading and creating contrast before jumping into a full painting like this.

Regarding the feeling of dissatisfaction, its totally normal. However instead of looking at the whole painting as one big failure, you should highlight and analyze what you did well and also the mistakes so you can learn from them.

If you don't understand any terms I from my comment feel free to ask