Have a painting that you are considering as a 'Failure'?  Have you tried to erase your mistakes? Several of things you can do would be:
Use the original 'Magic Eraser' that you find in your household cleaning supply areas of local retail and grocery stores.  This would work best in small sections of the painting. Just dampen the sponge and erase the area gently.
Scrub the painting by placing it in a bath tub with enough water to cover the painting and scrub the areas you wish to lighten or erase using a sponge. When finished place the painting on a board to lay flat while drying.