Your options depend on what the lease says about subleases or assignments. If your lease allows you to sublease or assign the lease, even if subject to landlord's reasonable consent, I would continue to advertise the space for a cheaper rate, and use that advertisement (which obviously undercuts landlord's attempt to get more money for the space) as a bargaining chip to get landlord to let you out of the lease early.

Think of it this way: you have the rights to the space for the length of the lease, not landlord. If the lease allows you to sublease, and you can do so for more than you're paying, you have an argument that you're entitled to the extra rent money. Landlord wants that money and may have to let you out of the lease to get it.

Another benefit of advertising the space is that by offering lots of qualified replacement tenants to the landlord, you can reduce landlords ability to come after you for the full extent of your remaining lease payments, etc. In those states which require landlord to mitigate damages.