What interest rate are you paying on the investment?
This is very profitable before interest and tax and still profitable after interest and tax?
The fact that it is in negative equity does not matter at all. To get a return of €2,000 a year after tax, you would need to invest about €50,000 in something other than a deposit account, so I would say that your current investment is worth about €50,000.
If the price increases to around €50,000 over the mortgage amount, then you should sell it.
Or maybe if the price increases to an amount big enough to clear your non-mortgage debts, then you should sell it.
Brendan