Sale deed of a flat not allowing for backing out of the project.

Hello Sirs, I had purchased a flat in Bhopal in December 2015, the agreement for which was made in January 2016. The builder initially promised possession by January 2017, which then extended to May 2017. We paid over 85% of the agreed value of the flat and also got it registered in October 2016. However, the construction slowed down to a snail's pace after June 2016, and possession is nowhere close in sight. Hence, we filed a case with the MP RERA and asked for the refund of the money deposited with interest as compensation. The authority, however, informed us that since we have registered the flat, it is not possible to cancel it and get a refund of the deposited money with the builder. This has left us in limbo as the other options available do not even cover the losses incurred due to non-completion of the flat, and we have completely lost faith in the builder to complete the construction well. In this regard, it would be appreciable to know what is the possible exit route in this regard.