What if we could shift the emphasis so that our exchange was aimed at identifying the elements of your attempt that are going in the right direction? The next step, then, could be to select one single feature of the painting that should be done differently.
For homework, then, paint the picture you were working on in class again, with an eye toward just one element that you want to change. For example:
The blue part of the church in the middle ground lacks substance. It's made out of jello. I want it to be farther away than the warm, dark buildings along the bottom edge of the page, and the blue color is useful for that, but it needs some kind of opacity or grittiness.
Proceed as if the rest of the painting is OK. By focusing on a single change, you increase the odds that you'll make a positive difference. I can certainly see other issues (those domes are too warm, or too dark?), but one thing at a time is enough.