once bitten, twice shy
Definition from Wiktionary, the free dictionary
Alternative forms 
- The burnt child dreads the fire.
- A beaten dog may cower before a friendly hand.
- A scalded cat is afraid of cold water.
- Bitten by a snake on one morning, afraid of the rope by the well for ten years.
one is cautious in the future if hurt in the past
- Gregory Y. Titelman, Random House Dictionary of Popular Proverbs and Sayings, 1996, ISBN 0-679-44554-4, p. 261.