If youve signed a new contract, which you have, and there is no break clause, then you are liable for the full term - 6 months in this instance. The fact that you have lived there for 12 months previously is, unfortunately, irrelevant as this is a new contract.
The contract could have gone periodic (i.e. a rolling contract with 1 months notice from you) if the landlord had agreed, but this is not the case. The landlord is not obliged to offer this.
Now that youve signed a new 6 month contract then that is what you are liable for. You can ask the agent to re-market the property but you will be liable for rent and marketing costs up until a new, suitable tenant is found, although this may well be the cheapest way out of it.