The distinction
Bitterness is a taste, registered on the tongue, driven largely by over-extraction and by heat. Astringency is tactile — a drying, puckering, rough sensation caused by polyphenols binding to proteins in saliva, which is why it feels like the mouth has been stripped rather than like a flavour. You can have either without the other, and diagnosing which you are experiencing is the first step to fixing it.
Different fixes
For bitterness, lower the temperature first — it is more temperature-driven than time-driven — then shorten the steep. For astringency, look at leaf grade and agitation before temperature: broken leaf releases faster, and stirring, squeezing a bag or shaking an infuser all increase it sharply. Very hard water contributes to astringency and not much to bitterness. Advice that offers one fix for both will solve half your problem.
Both are sometimes intended
Young sheng pu-erh from Bulang is aggressively bitter by design, and the returning sweetness that arrives seconds later — hui gan — is what it is drunk for. Matcha’s bitterness is structural. Dan Cong is brewed deliberately against the edge of bitterness because that is where its aroma is. Judging these by the standards of a mild breakfast tea misses what they are, and 'remove all bitterness' is not a universal goal.
Why the vocabulary matters beyond brewing
TeaHQ records these as separate axes on every tea and separate cause categories on every troubleshooting page, because a catalogue that collapsed them would give wrong advice systematically. If you take one piece of vocabulary from this catalogue, take this one — it makes every other piece of tasting advice easier to use.