Hi --- Unfortunately a contract cannot be legally terminated purely on financial grounds alone or the fact you may have found the same service cheaper, it can only be ended if they breach their terms of the contract with you i.e. they fail to substantially provide the service they should provide to you.
Therefore you cancelling your DD without any other good reason would place you in breach of the contract, this would mean that you were liable for the remainder of your unexpired contract plus any costs in recovering the money that they incurred or penalties written into your contract.
The best way is to contact them, explaining honestly..... that you're having great difficulty due to your new circumstances, to see if they are willing to reduce the cost or end the contract voluntarily.