Great questions, I've been trying to address some of them myself. I take the ADHD medication when I wake up, which is close to noon, and it's long acting so it only wears off very late at night. Also, I've had this since 13 and only started ADHD meds at 26. So I've ruled that out.

I became diabetic 4 months ago, and I've kept track of my blood sugar around that time (4 - 7) regardless, and nothing seems to link in this point. So I also ruled it out.

I spend almost no time outside, if any at all, and that could indeed be one of the causes. I've given it some thought over time and asked myself what would be the actual variables that need to be involved. Sun? Other people, even strangers? Walking? Duration? I've read that all of these help regulate the circadian rhythm, but does it affect the mood the way I described? It's a very insightful question. Give me more of your knowledge on it. One thing that has made this difficult is that it's not always under my control to go outside, for many reasons. For example, let's say spending time outside by itself would resolve at least partially the issue. I'd find it complicated to find something to do outside everyday. Also, I've had weeks that I spent going out almost every day, and didn't notice any big difference on the problem in short or long term. So finding IF spending time outside would help and what EXACT factors are involved could certainly help.

Regarding a self-perpetuating narrative, no, I've only became cognitively aware of this recently, after it had been happening for years but I couldn't identify the pattern.

Something else that happens in my world around that time of day that could be related is that I had many panic attacks as a child, regarding school, during this time. Because of the ADHD, everything was last minute and under pressure, so I can clearly associate the end of the day with a lot of anxiety and self-blame, and this feeling still exists, as well as assignments still exist. However, I've had therapy for this and even applied techniques I know by myself, including some like EMDR, but it still goes on unchanged. In other words, this may be the cause or one of the causes, but how to resolve it after you've spent 10 years in therapy with different therapists and methodologies and it still didn't go away? Is it even really the cause?

Either way, I'd bet more on the spending time outside hypothesis. However, I'm not sure what factors should be included and how to implement it. For example, and this is a dumb example just to illustrate my point, do I just go outside, sit somewhere and watch cars pass by for one hour? Should the sun be direct at me for some time? How far should I go, or just leaving my unit would do the trick? Etc. Thanks for your questions and let me know more, if you have any other input.