I suspect your coolant is going inside the engine. Head gaskets are notoriously hard to seal, and if the motor has gotten very hot, the head can warp. If you have not already done so, put on a new head gasket. If you have put on a new head gasket and not surfaced the head, invest in another head gasket and surface the head as follows.
When changing the head gasket, tape a piece of 400 grit sand paper to a window and surface the head in a circular motion using firm even pressure until the head is clean and smooth. Remove the studs from the cylinder and surface the cylinder as well. This is tougher because of the size of the cylinder, but take your time and do it right. This usually takes me about 40 minutes to do both the head and cylinder, so your arms are going to get tired, but this will assure you are not loosing coolant past the head gasket.
When reinstalling, use the stock gasket without any gasket sealer.
I assume you have checked your oil, but just in case...If the oil is milky, you have a bad water pump seal.
Good Luck.
Rick