Is this bitter or astringent?
Something in the cup is unpleasant and sharp, and you cannot tell whether to call it bitter or astringent. Both words feel roughly right.
Likely cause
They arrive together in most over-brewed tea, and they are detected by different systems, so nothing in the experience separates them for you.
Try this now
Swallow, then wait ten seconds without drinking anything. Bitterness is a taste and fades once the liquor is gone. Astringency is a texture and keeps building, so if your mouth feels dry and rough half a minute later, that was astringency.
What to change
Rate them separately from now on, even roughly. For either, lower the water temperature before you shorten the steep.