You have not forgotten it. That is the confusing part. It arrives at eleven at night, in the shower, in the middle of something else, and it has been doing that for weeks.
So the problem is not memory, and every tool built on the assumption that it is memory has failed you. Writing it down again does nothing, because it was never off the list.
The gap is between knowing and starting
Starting almost anything administrative requires holding several things at once: what this is about, what you want to happen, the two facts you will need, and enough spare capacity to think while somebody talks at you.
That is a working memory load, and it is heaviest at exactly the moments you tend to attempt these things, which is late, tired, and already carrying the day. The task is not hard. Assembling the conditions to begin it is.
The two questions that unstick most things
2 things to do
Ticks are for this visit only. Nothing here is saved anywhere.
The thing gets heavier the longer it sits, but not for the reason you think
The task itself does not change. What changes is that it acquires a story: that you have avoided it for a month, that this says something about you, that starting now means admitting to the delay.
That accumulated weight is not part of the job. It is worth naming, because it is usually the larger of the two things stopping you, and it is the one that disappears the moment you do anything at all.
Aim for a smaller thing than finishing
Finding the reference number is progress. Getting through the opening sentence of a call is progress. Neither finishes anything and both remove the part that was actually blocking you.
If the thing is a call you have been dreading specifically, the preparation that helps is set out in making a phone call you have been avoiding.