How big is your tree? How big is the pot?
Drooping leaves is a sign of lack of water reaching the leaves. This can be due to dry soil, or root damage which can be caused by overwatering. So you have to work out which it is.
Are you absolutely sure that the soil is damp throughout? Sometimes a full leaf canopy prevents much rain reaching the soil surface, even after heavy rain. So the top layer may be damp but the rest bone dry. It can be difficult to wet certain dry soils - water just flows through without wetting. Dig down a bit to check. This is easy to remedy - just stand pot in water for several hours.
If the problem is root-rot due to overwatering you have a worse problem. You could try re-potting in drier free-draining compost while cutting off any rotten roots. The leaf canopy should be reduced at the same time by pruning as the damaged roots can't support so much. You could try treating soil with the French fungicide Aliete. And prevent further water reaching the surface until it dries. Or you could just hope it improves as summer approaches.
Fertilising won't do any good until you have sorted the drooping leaves.
Also check for any root-eating pests.