Refund from builder

I purchased an apartment in Mumbai in March 2019 . RERA date was Dec 2020, But builder promised to deliver by March 2020. My mistake was that i did not check whether full cc was received ( which wasn't). FIat cost was Rs. 1.8 cr. plus GST and stamp duty/ registration. I took loan from HDFC bank of 1.38 cr. and bank paid 90 lakhs to the buider as per progress payment terms of the agreement after verification. . My pre-emi interest started in April-2019 ( almost 68,000 Rs.). In May, 2019 Work stopped at site ( due to financial issue of builder). Then came COVID in March 2020 which further delayed the project . In October 2020, work started again and builder assured to give me possession of my flat by September-2021, but he said that the rest of the project ( other wing plus amenities) will take time uptil June-2022. He gave me a letter in writing to that effect . But even that commitment was not met and work stopped again in March 2021. It is now June-2022, and work is still ongoing . The builder is now selling the remaining flats with possession promise of Dec-2022. I have till now paid pre-emi interest of almost 20 lakhs, which is a big amount . Can i demand this amount back from the builder (or atleast the Pre-emi amount after Dec-2020, which was the original RERA date in the agreement)? I have yet to pay the balance 15% to the builder ( approx 27 lakhs) and have written a letter to the builder 3 months back to consider deducting the pre-emi amount from he balance 15% , But there is no response from him. Can i demand my entire amount paid to builder back along with interest, if the builder does not agree to my letter? How can i get him to reply to me? I have so far paid 70% of the flat cost to him and paying a heavy pre-emi every month ? What legal action can i take against him? losing the flat is the last option, but if the builder does not agree , then i have no option but to let go of it, but how can i get my money back legally including the pre-emi interest and stamp duty i have paid.