"The most important ally in the study of painting is the art of thinking." Edgar Payne.
Ask yourself the following questions when analysing the success of your paintings...
Is it finished?
Has it been overworked?
Is it too fussy?
Does it say what I felt?
What could I soften?
What could I leave out?
What could I accentuate?
Is it balanced?
Could I crop it to make a better composition?
Does it look like a picture made with watercolour?
If you feel something is wrong but you're struggling to work out exactly what it is looking at it in a new light may help. Try the following...
Put a mount (mat) around it, move it around to see if the problem is compositional.
Look at it in a mirror.
Look at it upside down.
Photograph it in black and white.
Scan it onto your computer and use a programme to alter colours, tones, format etc.
Prop it up somewhere where you will see it in your peripheral vision.
Put it away for a while and re-evaluate it at another time.