Why does my cocktail taste like nothing but alcohol?

Under-dilution, wrong proportions, or both. Most likely you've under-stirred or under-shaken (the ice hasn't melted enough), used too much spirit relative to modifiers, or used a low-quality spirit that's harsh straight.

If a cocktail tastes like a shot of whiskey rather than a balanced drink, three likely problems: (1) Under-dilution. A Martini stirred for 8 seconds is half as diluted as one stirred for 25 seconds, and the alcohol burn dominates. Solution: stir or shake the full recommended time, with proper ice. (2) Wrong proportions. Cocktail recipes are mathematical: 2:1:0.75 (spirit:citrus:sweet) for sours; 1:1:1 (gin:Campari:vermouth) for the Negroni; 2:1 (whiskey:vermouth) for the Manhattan. If you've poured spirit free-hand, you've probably poured more than the recipe calls for. Use a jigger. (3) Spirit quality. Cheap spirits taste like alcohol because they ARE more like alcohol — fewer congeners, fewer aromatic compounds, less to integrate with the modifiers. Upgrade the base bottle to mid-tier ($25 to $40 range) before changing anything else about your technique. (4) Spirit choice. Some spirits dominate cocktails more than others. Vodka is the most likely culprit because it adds strength without flavor — a vodka Martini at 3 oz vodka and 0.25 oz vermouth tastes like vodka with a hint of vermouth. Increase the vermouth or back off the vodka.