As per law laid down by the Supreme Court and the National Consumer Dispute Resolution Commission (NCDRC), you are under no obligation to accept possession of the flat beyond the stipulated time period and can ask for full refund alongwith interest and compensation. This is settled law and UP Rera cannot compel you to take possession. You can refer to the following cases in your support:-
1. Emaar MGF Ltd v. Dilshad Gill (2015) CPJ 329 (NC) by the National Consumer Commission
2. Emaar MGF Land Ltd v. Dr. Manuj Chhabra, FA No. 1028 of 2015 decided on 19-04-2016 by the National Consumer Commission
Alternatively, you can also withdraw your complaint by making the said request - "Withdraw with liberty to file afresh before appropriate forum". Once you have withdrawn the case, you can file a fresh consumer complaint before the State Commission or the National Commission, depending upon the value of your property, for refund alongwith interest and compensation.