The ending can be locked in from the beginning, if you follow the advice of artists such as Andy Evansen who always work in three stages. This avoids overworking and fiddling. Once you have gone through the stages you are done.

Of course you may be less than satisfied with your results, but that is just down to practice. So with this mindset you are less attached to individual paintings and more focused on your artistic development.

If im less than satisfied with what ive done, which has actually increased lately as my ambitions get higher, i just flip over the sheet and use the other side aswell. I have plenty of paintings ive made that im pleased with, that have an earlier version on the other side.

I would add that finishing a watercolour is kind of determined for you as you know when you are overworking it. With oils of acrylics you could go on fiddling forever.