Its important to be able to constantly improve your work. It's no good saying "I don't like that it's not working" if you can't take the next step and assess what is wrong.
If you finish your art work and something is wrong, do not just bin it.
- Look at it from a distance - we mostly paint too close to the paper and it's amazing what changes if we look from a distance, or in a mirror
- Put a mat/frame around the painting - maybe it's not as bad as you think.
- Crop the painting differently sometimes in a smaller painting the focal point changes
- Try adding darks, your painting may just be missing tone 5 (in the foreground not the background)
- Put it somewhere you can look at it for a couple of days to reflect
- If it really is beyond repair use the back of the paper - you are guaranteed to get a masterpiece as psychologically your paper is now valueless and you probably find that you are painting in a much freer style!!!