Speak to your bank before handing back the keys. My bank Caxia Galica organised a two year fixed rate remortgage for me, bringing my monthly repayments down from 800 euros to 350 euros. I am presently out of work, so am still struggling to find this amount but it is sure better than 800 euros. If you can find any way of keeping your flat I think you should because ultimately the consequences of you giving your keys back to the bank could be far worse in the long run than keeping your flat and riding out the storm.
Best of luck!