Disclaimer: The characters and situations described herein are the property of Joss Whedon and Mutant Enemy. Take no offense and hire no lawyers.
He fills his life with empty vessels, because if he surrounds himself with them—conditions himself—then maybe eventually he'll learn to believe that was all she really was. Which seems easier, most days, than what he actually feels—the aching pang of the absence of her. Like he's missing a piece of himself. A world without Cordelia is a world without a right arm, or a sense of smell—or a beating heart, but he's been living without one of those for two and a half centuries, and he's sick and tired of being defined by what he lacks.
There's a kind of pathetic kinship, now, between him and the phantom Dennis. Which is ridiculous; and how cruel was she, anyway, to live that kind of life—to light up the existences of men long dead, make them feel and hope and die all over again when she left them just as suddenly as she'd come.
Harmony helps. He'd agreed to hire her solely because she was one of the few people he knew worse off than himself—at least he has a soul, something quantifiable with which he can measure his worth. He hired her because she was empty, and he keeps her around for much the same reason. The habitual tactlessness, the flaky but well-meaning attempts at office management, the blood spiked with cinnamon and good intentions… it's hollow and meaningless, and yet so very like her, how it was that first year. He trains himself to equate them, to transfer annoyance at Harmony into annoyance at her. It helps.
And if he often feels abandoned and directionless, it's because he no longer has her visions to guide him, and not because he no longer has her. He'd asked for neither best friend nor Seer, and she had been both; but since the Powers That Be seemed to think he could get by just fine without either, that's what he would do. If he sometimes hates Eve for being so very young and so very annoying, it's because at least Lilah Morgan had been a worthy adversary; it has nothing to do with the few small mannerisms she shares with the girl just out of high school who'd grown into such an extraordinary woman that he'd sometimes thought of them as two separate people. If Wesley still hasn't learned to argue with him properly, after all these years—pushing when he should leave be, ignoring when he should protest—well, Angel was never that talkative to begin with, and it had taken him ages to find her endless chatter soothing. He can learn to appreciate the silence again; he doesn't need a cheerleader. (Funny, in the way that isn't, how he always manages to fall for them anyway.)
So he builds a vacant world to match his vacant chest: an empty vessel at what should have been her desk to match the empty vessel in a hospital room he still hasn't been able to bring himself to visit.
And when that doesn't work, he files her away in the darkest corner of his mind; she grows dusty and forbidden-not-forgotten next to a Claddagh ring, a shanshu prophecy, and a disassembled crib.
The problem with nothing, he thinks, is that it never feels like something—no matter how much of it he collects.
A/N It always bugged me how Cordelia was some kind of off-limits topic after she fell into her coma (barring "You're Welcome"). Especially considering the fantastic lengths the boys went to when Fred was in danger. This was me, making peace with canon.