Once bitten twice shy

[Britney]

orignally to dog bites, now means that once somebody has gotten hurt or had something go wrong will be afraid to try it again (especially in love)

If you fall in love with someone and that someone rejects or hurts you-once bitten-you may be very, very reluctant-twice shy-to let yourself fall in love again.

[Little_Girl]

This is an abbreviated sentence:

Once your are [bitten] by a dog, you will be twice as shy about [approaching] that same dog again.

Metaphorically, the concept [applies] to more than just dogs. Such as cats.