Why is my Old Fashioned bitter?

Too many dashes of bitters, or too long a stir without enough sugar to balance them. Two to three dashes Angostura is standard; more than that, you tip into bitter.

The Old Fashioned is whiskey-forward, sweet-leaning, and aromatic — not bitter. If yours tastes bitter, three common causes: (1) you've used five or six dashes of bitters instead of two or three; bitters are concentrated and the difference is real. (2) You've used demerara syrup or sugar paste in too small a quantity to balance the bitters; classic spec is one sugar cube or 1 teaspoon (5 mL) of demerara syrup. (3) You've stirred for too long over too much ice, diluting the whiskey out and proportionally amplifying the bitters' presence. Fix in this order: cut bitters back to 2 dashes, confirm you're using a full sugar cube, stir 15 to 20 seconds over one large ice cube rather than crushed ice. A correctly built Old Fashioned tastes like whiskey with depth, not whiskey with a medicinal layer.