make me stop!

This is Jessie, my son's 5 year old minature fox terrier. I love her colouring , black and tan and white, and I wanted to try and capture some of her charachter in this watercolour sketch. However I've overworked it as usual. I pass through the phase of thinking it looks pretty good ....and make one brushstroke too many so that I spend an anxious 15 minutes trying to save what I had originally.

Why can't I just learn to STOP when it's done, instead of fiddling and labouring and spoiling things?

I'm happy enough with it now, but it's lost the freshness I saw earlier.