If you've already signed the lease, you're under contract and obligated to the landlord for the full term of the lease period. However, the landlord has an obligation to mitigate his damages, which in this case means that he would have to actively and diligently pursue another renter to lease your space. If the landlord is successful, then you would only be liable for the time that the space was not rented. For instance, if the apartment is vacant in June because you didn't move in, but the landlord rents the space in July to another tenant, then you would only be obligated to pay one month's rent.

You should give your notice in writing so there is no question of your intentions. You should also ask the landlord to actively pursue another tenant to mitigate any damages caused by your breach.

The worst case scenario is that the landlord cannot rent the space and you're on the hook to pay for the entire period of the lease - this is highly unlikely. The most likely event will be that you have to pay for a few months rent until the space is re-let.

However, because you haven't even moved in yet, it is possible that the landlord could find a new tenant over the next 1.5 months, and there would be no harm to anyone.