An old quote from my archives. I don’t even know who Hari Kunzru is, but he sums up my thoughts on the matter perfectly:
I get great pleasure from writing, but not always, or even usually. Writing a novel is largely an exercise in psychological discipline – trying to balance your project on your chin while negotiating a minefield of depression and freak-out. Beginning is daunting; being in the middle makes you feel like Sisyphus; ending sometimes comes with the disappointment that this finite collection of words is all that remains of your infinitely rich idea. Along the way, there are the pitfalls of self-disgust, boredom, disorientation and a lingering sense of inadequacy, occasionally alternating with episodes of hysterical self-congratulation as you fleetingly believe you’ve nailed that particular sentence and are surely destined to join the ranks of the immortals, only to be confronted the next morning with an appalling farrago of clichés that no sane human could read without vomiting. But when you’re in the zone, spinning words like plates, there’s a deep sense of satisfaction and, yes, enjoyment…
Brian D. Buckley 
I read about this a lot, the highs and lows, the self-disgust and self-congratulation, and I confess I don’t get it. “Get” in the sense of experience, not “get” in the sense of understand.
I have never felt like I was ever going to join the ranks of the immortals, and that’s never bothered me. I’m not saying that to boast (I’m missing the highs as well as the lows, apparently) or to complain — it’s just an observation. Maybe it’s just my personality, maybe it’s because I’ve been doing it for a while (I’ve been writing pretty seriously for nearly 45 years), maybe it’s because I’ve never tried or wanted to make a living at it.
Based on what I read, I’m very much in the minority, but that’s often the case. I remember watching TV shows with my ex, knowing that, inevitably, the more we liked a show the more quickly it was going to get canceled. 🙂
I think there are a lot of writers in both camps: the high-and-low types, and the steady-as-she-goes types. From what I understand, Isaac Asimov was firmly in the latter category. So you’re in good company. 🙂