Unfortunately, legally, in Massachusetts, landlords are not required to mitigate damages. So, you can be held liable for the rent, at least until Ll finds someone new.
Here's the link to the law, use your search feature to find it (control F) then put in Massachusetts:
http://dirt.umkc.edu/files/mitigationsurvey.htm
So, your best bet is to try and work out a deal with the landlord. Unfortunately, the law is not on your side. But, if you learn the LL has rented to someone else, you can sue under the premise the LL is not allowed to double dip (collect rent for the same period from 2 different renters).