The apology that gets added on day four, then day nine, growing in proportion to a delay it is also making worse.
You would happily talk to this person for an hour. They sent four sentences three days ago and you have not answered.
It is not reluctance. If they rang right now you would pick up and it would be easy. Something about the reply specifically is harder than the conversation it is standing in for.
A spoken answer is assembled while you say it. You start a sentence without knowing how it ends, the other person's face tells you whether it landed, and you adjust. Nothing is recorded. Nobody rereads it.
A written reply is a small piece of published work. You compose it, you can see it before sending, and once sent it sits in a thread that both of you can scroll. There is no face to read while you write it and no chance to adjust mid-sentence.
So the same content costs more. Not because you care more about the writing, but because the medium removed the two things that made speaking easy: the feedback and the forgetting.
In conversation, things fall away. Someone asks three questions, you answer the interesting one, the other two evaporate and nobody minds.
A message stays on screen with all three visible. Answering one and skipping two looks like a choice, because it is one. So a reply that would have been a sentence out loud becomes an audit of everything they said, and the effort of doing that properly is exactly the effort you do not have at 9pm.
This is the part that compounds.
An hour after it arrives, a message needs an answer. Three days later, it needs an answer and an explanation for the three days. A fortnight later it needs an answer, an explanation, and a tone that acknowledges the whole thing without making it a bigger deal than it is.
The reply you owe grows while you do nothing, and the thing that makes it grow is only ever the delay. Which is why "sorry for the late reply!!" gets longer and more apologetic each time you nearly send it.
Answer the smallest part. You do not owe a complete response. One line about one thing reopens the thread, and the rest can arrive later or not at all.
the second thing, yes. the first thing is a longer story
Skip the apology, or make it one word. A paragraph about why you were slow makes the delay the subject of the conversation and asks them to reassure you about it. "sorry, slow week" is plenty. Often nothing at all is better.
Send the reply you would have said out loud. Not the one you would have written. Those are different lengths, and the spoken one is almost always the right one.
If it has been very long, name it once and move.
this is three weeks late and I have no defence. anyway, yes to Saturday
Some messages never get replied to, and it is rarely because the sender did something wrong. It is that the reply was hard on the day it arrived, then harder, and at some point answering would have required addressing why you had not.
That is worth knowing about yourself, because it looks from the outside like an opinion about the person. It usually is not one.
There is a related piece on the other side of this, when the message is one you want to send rather than answer: I want to text my friend, but I don't want to bother them.
An unanswered message is almost never about the person who sent it. Knowing that does not make the reply easier to write, but it does make the delay easier to end.