Separate what you know from what anxiety has added.
What you know might be one sentence: I sent a message at 2:10pm and there is no reply yet. Everything after that may be interpretation. They are losing interest. I said too much. They saw it and hate me. I need to fix it.
Write the facts and the story separately. That does not prove the reassuring explanation is true either; it simply stops the worst-case explanation being promoted to fact without evidence.
Notice the behaviours you use to make the uncertainty disappear.
Checking last seen, rereading your message, watching story views, asking a friend what they think, sending another message or changing the subject can all produce a brief feeling of action. The problem is that the relief can teach your brain to demand the same checking next time.
NHS information on health anxiety gives a useful parallel: repeated checking and reassurance can become part of the cycle that keeps anxiety going. The subject is different, but the principle of noticing and gradually reducing reassurance behaviours can still be useful.
Create a no-chasing window before the anxiety arrives.
Decide a rule when you are calm: I will not send a follow-up purely to relieve anxiety for a set amount of time. The exact number depends on the conversation. You are creating a pause, not playing games or deliberately withholding communication.
If there is genuinely urgent practical information, communicate it. The distinction is whether the second message is solving a practical need or trying to make your body feel certain again.
Look at the relationship pattern, not the response time of one afternoon.
Healthy connection is not measured by instant replies. Look at consistency across time: do they communicate, follow through, show care and make room for you? One slow reply cannot answer that question, and one fast reply cannot answer it either.
If somebody repeatedly uses silence to punish, control or destabilise you, that is a different problem from ordinary reply anxiety. Pay attention to the broader behaviour rather than trying to diagnose it from a read receipt.
Give the waiting period somewhere else to go.
Do an activity with a clear end: shower, gym session, walk, meal, game, chapter, house task. The aim is not distraction forever. It is to stop the message thread being the only object your attention is allowed to return to.
If writing helps, record the thought you are having and the evidence for and against it. A simple journal can support the exercise, but the useful part is challenging the certainty of the story.
If reply anxiety repeatedly controls your relationships, get support for the pattern.
You do not need to wait until a relationship breaks down before taking the anxiety seriously. If reassurance seeking, panic or fear of abandonment is dominating your behaviour, talking therapy can help you understand and change the cycle.
Lifted Spirits can give you other people to talk to and somewhere to reflect before acting, but it is peer support rather than therapy.