100% mortgage was your own choice so you presumably assessed the risks at the time? Whether or not you have "made a mistake" remains to be seen. But negative equity is largely of academic interest as long as you can service the mortgage repayments and are not planning to sell up for the forseeable future. If you had taken an 80% mortgage, stumped up the 20% deposit yourself and sold later at less than what you paid you would still make a loss (albeit without having to pay interest on the additional 20% capital and having already secured ownership of a chunk of the equity in the property).