Your going to lose money whichever way you do it, trade it in or run it.
Seems everyone has gone green all of a sudden with many great diesel cars becoming the scourge of mankind even though, with particulate filters and ad-blu, many diesels are cleaner than their petrol equivalents.
There was an article some months ago comparing a 44t Mercedes truck to a 1.0 VW polo and the truck was cleaner, much cleaner.
Personally, unless you want to run newer cars, I would keep the one you have got.
Better the devil you know.