Character Role Analysis
We could have put something abstract in here, like "fate," "chance," or even "time," and that would have been close—but the truth is that Tristram is his own worst enemy. He's constantly getting distracted by digressions; he's forever bemoaning the difficulty of telling his story, and his bodily infirmities make everything worse (he's sick all the time).
But you could still argue that there's no real antagonist and that those crazy abstract forces are working against all the characters in the novel, and you'd have a pretty good case. Call it Tristram or karma—none of the characters are ever quite able to get what they want.