Keep a log of your brew parameters. Write down grind setting, dose, yield, and time for each shot. When you hit a sweet spot, replicate it. Also, buy fresh beans in small batches and use them within two weeks of roasting. And don't forget to descale regularly.

Bitter espresso is fixable—most of the time it's just a matter of tweaking one or two variables. Start with the grind, then move down the list. You'll be back to smooth, balanced shots in no time.