Answer
It is clear that it was not permissible for you to purchase a flat on an interest bearing loan. You should sell the flat and come out from the interest deal even if you suffer a loss.
However, if the loss is absolutely unbearable, then you may keep the flat and also make Tawbah and Istighfaar for being in an interest deal. What if death comes to you while you are involved in this deal which incurs the anger and wrath of Allah on you? It is also advisable to examine your cases of ?absolutely unbearable loss? to a reliable local Mufti and follow his rulings in your matter.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai