The Categorical Imperative fails for me because it does not matter to me whether everyone lies when they wish or not. I place no moral value on not-lying, just as I place none on lying. There is literally nothing I can "will" to become universal law.
I have a similar problem with theological sources of morality. Such codes rely upon obedience to a higher power which will punish you if you do not obey, and reward you if you do. What fear have I of punishment? What desire have I for reward? It breaks down immediately once you remove emotion from the equation.
Secular humanism is as much a fool's errand as theology. Such a philosophy presupposes the "specialness" of humans, the need to regard all humans as equal moral agents. What basis the specialness of humanity? Sapience? Perhaps. If humans are the only sapient life form in existence, as we are on Earth, then perhaps we are special. And if our mission discovers sapient alien life, as many hope? Humanity's specialness diminishes.
Some have suggested I would be a good nihilist. Perhaps. In some ways, my non-beliefs are in line with the tenets of nihilism. Or they would be, if I could accept a "non-belief" as a thing akin to a belief. Nihilism, however, seems to involve in most of its adherents a sort of world-weary cynicism that I cannot truly comprehend or explore. Even in those who do not feel this way, to embrace a philosophy, to append its name to yourself, requires an effort of recognition and sympathy with its views that I cannot perform.
Ultimately, the quest for moral understanding may be utterly futile. No philosophy can truly be formed and followed in the absence of emotion. But then again, there is also the possibility that, if there are aliens out there, they may have solved the problem themselves. That supposes there is a problem, of course, but one must make certain presuppositions to enable any course of action.