There are two separate issues really:
1. The OP is sensitive to tax, so really should be driving something of 1.4 and under. Ideally maybe 1.0 or under - but if motorway miles are done then that may not be possible.
2. Mondeo's depreciate. A lot. I noticed this myself when looking at cars recently, demand for 'big' cars just isn't what it used to be.
Only you can decide what to do, OP. Try to get things down on a list as to what the plues and minuses are. If the tax is going to force you to struggle then perhaps getting rid of the car is the best option, at whatever price you can get. You could try and swap, there may be someone out there looking for a bigger car.
Really though, your best option may be to go for an older, smaller car with a more frugal engine. It will be a downgrade, but needs must.
It's scary to think I sold a 2000 Yaris recently for €2,100.