BBJF,
Sorry for your loss . . . I have an Eastern long neck too. They are a great pet. When I get back from fishing and am filleting fish at the kitchen sink, from his tank in the next room he knows what's coming up and is literally climbing and stamping on the water to get his fish bits. Anyway, I don't want to bring back too many memories for you.
As far as feeling the loss more . . . well, I guess it comes down to the sudden-ness of his passing, and maybe you just loved him a little more than you thought.
If he was sick and the vet said he's got 3 months . . . you kind of prepare yourself for it. But if it's unannounced and especially premature, it hurts more.
And like most have said, a pet gives itself unconditionally. A trait hardly found in us mere mortals.
Chin up mate.