hello i have an issue where i possess the body of a young green dragon using the greater possession spell, but then when fighting a lich he use the energy drain spell and the gm express to me that the spell drain both the dragon and my levels. so my question is is this possible?
There is nothing to cover this as energy drain isn't a regular form of damage and the possession and equivalent spells only cover normal damage and ability damage/drain (which goes to the possessed body). While you might try to argue that energy drain comes from an attack to the body in most cases, so it should likewise go to the normal owner of the body, energy drain is a spiritual attack that essentially damages the soul within the body (hence why many incorporeal undead have it as well as corporeal).
This is complete speculation and personal interpretation, but for a possession spell where the soul of the body is essentially just trapped in said body while you take over from inside and isn't sent away like Magic Jar, I think it would be fair and reasonable to essentially treat it like "cleaving" both souls and drain the both of them.
There is no rule to cover this but I'd say physical attacks affect the body of the possessed.
Mental attacks affect the mind.
If you use magic jar as a starting point, which is reasonable at no point are the mind and body attacked by physical attacks which is what energy drain would be.