Is it correct that you are based in the UK and that you are able to afford 500 Euro a month without rent.
Forget about how the bank are treating you, they're all like that.
Your mortgage is clearly unsustainable, based on the fact you have had interest only, payment breaks and reduced payments.
So selling is what you need to do. I would focus on that only. Chase them to agree to the sale for 60K. Be very careful that you agree with the bank that the legal and auctioneering fees are paid out of the sale proceeds, (unless you can afford them). You will probably have to sign a document stating that you are liable for the shortfall (which you are anyway) sign this and then you negotiate, after the sale, on this. This negotiation will be either you want them to write it off, or that you pay it back at the same mortgage interest rate and term you are currently on, or that you threaten UK bankruptcy or that you just decide not to pay them anything if they won't negotiate. If you can afford to pay it then you should pay it, if you cannot afford it, have no assets, and a low income they cannot get much out of you. They may just ignore the shortfall and never chase you, depends on your circumstances - and yes banks are ignoring shortfalls in certain situations.