Farmer cheese turns out inconsistently
Based on lots of experience making farmer's cheese (and backed up by the book One Hour Cheese), there are three factors which control the scale of creamy to dry &amp; crumbly:


the degree of acidity at which you set the curd;
the length of time you cook it (keep it at 180F+)
the length of time you drain it


So my guess would be that you increased two out of three of the above, or all three, without intending to.  For example, you might have added the whole amount of vinegar instead of halving it, or, because you were draining half as much, it could have dried faster. Note that all textures from gloopy to sawdust are "legitimate" farmer's cheeses, and in Russian delis you can buy cheeses with the whole range of textures, for different applications.

Also, your cheese is not unsalvageable.  Depending on what you're using it for, you can add a little cream, sour cream, or full-fat yogurt to make it creamier.  And for that matter, dry and crumbly is perfect for making blintzes.