[CREWZIN]

That's really a personal decision. If you don't want to spend any more money, trade it in. If you want to find out what's wrong, a compression check and cylinder leak-down test should tell you the condition of your engine and you can go from there.

Losing as much water as you say, without a noticeable leak is not good news. But it could be something you overlooked like a pin-hole in the radiator hose which is leaking onto a hot engine/exhaust part and evaporating/burning off before it hits the ground.