So I see nothing wrong w/ blaming myself. But having said that, knowing the cause of death is most likely GI stasis, did you find out what cause the GI stasis, because on that link, it says:
(1) stress, (2) dehydration, (3) pain from another underlying disorder or illness (such as gas, dental problems, infections, or urinary tract disorders) (4) an intestinal blockage or, (5) insufficient dietary crude fiber
So find out whether it is (3) (4) or (5), and get to the bottom of it. That's what I would do.
Very sorry to hear your rabbit passed away so young.