Why is my espresso bitter?
Bitter espresso is usually over-extraction: grind coarser, shorten the ratio, or lower the brew temperature. If none of that helps, the bitterness is coming from the roast, stale beans or a dirty group head rather than your recipe.
Over-extraction is what happens when water keeps dissolving material after the sweet stuff is gone. It shows up as ashy, drying, lingering bitterness, often with a hollow or burnt finish. The fix is the mirror image of the sour fix: grind a couple of steps coarser so the water moves faster and stops earlier, and check that you are not simply pulling too much liquid. A shot that trickles 45 g out of an 18 g dose over 45 seconds is extracting long past the point of pleasure.
Ratio deserves its own look here. Longer ratios extract more. If you are running 1:3 on a dark roast because someone said long shots are sweeter, reverse it: dark roasts are already soluble and fragile, and they usually want 1:1.5 to 1:2 with a shorter contact time. Light roasts are the ones that benefit from being pushed longer.
Temperature is the other big lever, and it is underused. Dark roasts scorch easily. If your machine has PID control, drop to 88 to 91 °C for a dark or medium-dark roast and taste again. On a machine without temperature control, you can approximate the same thing by pulling sooner after the heat-up light cycles, or by flushing a little water through the group to cool it on a heat exchanger.
Now the causes that no recipe change will fix. Old coffee tastes flat and bitter no matter how you extract it, and beans past about six weeks off roast are in that territory. Rancid oils in a dirty machine are worse: a group head that has not been backflushed in months, or a portafilter basket with a film of old coffee oil in the holes, will add a stale bitterness to a perfectly good shot. Take the basket out, hold it up to a light and see if the holes are clear. Run a detergent backflush. This is the single most common hidden cause of bitterness in a home setup that used to taste fine.
An opinion, clearly labelled as one: a lot of what people call bitterness in dark roasts is not an extraction defect at all, it is the roast. Roasting past second crack develops carbonized, bittersweet flavors on purpose. If you are chasing sweetness and clarity and your beans smell like a chocolate bar and look oily, no amount of dialing will get you there. Buy a lighter roast and start over.