From your description the situation looks hopeless. You can only hope for some diplomatic miracle, but I can't imagine why any of the opposing players would risk angering his allies no matter what you offer.
Generally, this situation (getting a big alliance going against you) comes sooner or later if you're winning. Good diplomacy can only delay it or make it less synchronized. Fighting defensive war in such circumstances is very difficult (unless you're undead Ermor or water nation). I was usually going for the opponent I could conquer quickest. Don't bother about provinces much, take his capital and/or other fortresses. The time is critical here - you're racing against them destroying your income. Then go for the next opponent etc... They will still hurt your economy pretty badly, but at some point you'll be able to start taking provinces back. It looks you could have managed it if you went for it right from the start of the war; now it seems unlikely to happen, but if the map is small you can still try. Besides, when the opponent loses most his army and crucial fortresses and faces total obliteration he might be more open towards peaceful talks
