Why do I crash after only two productive days?

I've just had two really good, optimistic days. I planned following adhd-frendly tips. I slept well. My diet has been great: simple, varied, healthy meals that are easy to prepare. I did my grocery shopping in advance, and I have my meal plan written down.

For the past two days, I've been waking up early feeling rested and mentally clear. I'd do about 30 minutes of admin work, then go to the café next door and sit on the terrace for an hour to work on a long administrative task. The rest of the day was dedicated to a big drawing project for a friend, with a nap after lunch, a bit of tidying around 4 p.m., and some exercise. Once I was tired in the evening, I'd switch to another project or just fully relax by playing video games or calling a friend.

It honestly felt great. I was in a good mood, my mind was relatively clear, I was making progress at my own pace, and my body felt satisfied. I did have a "routine," but it wasn't built around strict schedules. It followed my needs much more than the clock (for example, eating or taking a nap when I genuinely felt I needed to).

I started thinking I'd finally found the perfect structure for when I'm working on a big solo project like this drawing.

But today, when I woke up, something felt completely different.

All I had planned was 30 minutes of admin work, a medical appointment, a few errands in the morning, drawing in the afternoon, and seeing some friends tonight. And yet I woke up with this awful feeling of heaviness. Just thinking about those 30 minutes of admin work felt suffocating. Everything I had to do suddenly felt enormous. It was like I was a pressure cooker about to explode. I could feel it as the warning sign that I was about to blow everything up and fall back into a completely chaotic rhythm.

Ironically, just like the previous two days, I was incredibly efficient all morning. There was almost no cognitive friction between tasks because of my organisation. But I could also feel that this efficiency was slowly draining my reserves.

After lunch, I started doomscrolling. When I forced myself to stop after 30mn, I completely crashed. I was so exhausted that I fell asleep until 5:30 p.m. I had absolutely no desire to get out of bed.

So now, in about an hour, I'm going to see my friends, and I've made exactly zero progress on what I was supposed to do this afternoon, which is a big project i love to do.

Why do I seem to pay so heavily for just two productive days? Why do I get this feeling of oppression when my routine is actually pretty flexible? Why do I feel so overwhelmingly tired even though I'm sleeping well? How do you deal with the feeling that you're about to crash? Why can't I even manage two reasonably productive days in a row like a normal human being? It's not like i had a lot to do, i can't reduce the amount of things i do in the day (sleep, eat, shower, clean a bit, important stuff 1, important stuff 2, entertainment, sport optionnal)

If anyone has any thoughts or explanations for what's going on, I'd genuinely love to hear them.

I'm also really scared that tonight will turn into the usual pattern where I end up staying awake until 4 a.m. because my brain suddenly becomes overexcited and then lets gooooo for 4 days of full chaos !

(More info: I'm not on medication because stimulants aren't an option for me due to a heart condition.)