The word someone needs may arrive after the conversation has already moved on.

Feeling and speech run on different clocks. The right sentence often assembles itself an hour later, in the car, after the subject has closed and everyone has moved on to something else. Losing it feels like having missed the only chance, which is rarely true. Most things that mattered enough to keep thinking about are still worth saying late.

Send the message anyway, hours or days afterward, without apologizing much for the delay. Say plainly that you kept thinking about what they told you. Arriving late is nearly always better than not arriving, and it carries proof that the thing stayed with you.