Over the years, I adopted a few tools to help me manage what feels like "open drawers" in my head :

1. When a task I need to do spontaneously pop in my head and it's not urgent, I write it on my whiteboard on my wall. If I'm not home, I quickly put it in my notes app. Then, and it's the crucial trick, I forget about it. That way, I can focus on the task(s) at hand and just organize my future tasks later, instead of juggling with them all at once.
   This helped me compartimentalize a lot and I found myself being able to feel free during the day here and there and on most nights, instead of being a sack of anxiety 24/7.

2. When I consolidate my tasks, I use a card system like Trello. (I don't recommend Trello specifically, it seems to have enshittified.) I keep it mostly pen-and-paper, but I'm sure there are some FOSS tools you can find that do just that.
   A card system was a revelation for me, being able to do a quick assessment of when a task is due and put it in the right column was a game changer. I'm the type to do everything at the last minute, so putting up little deadlines everywhere helped me do more.

3. This could not be helpful for you, but I take ADHD medication. The first one I tried for a few months, but it made me even more anxious. I then stopped taking anything, but decided after a year to try another molecule. This time, I found my dosage and it definitely worked.
   Before that, even with my calendar, tools, alarms and support system, there were weeks to months where I felt lost in thought, apathetic and disorganized. Now I can post a lengthy answer to a topic close to my heart without feeling like I'm not doing enough with my day.

Side tips :

• If I can break down a big task into smaller ones, I do that. That way, they never feel insurmontable. Plus, every few minutes I can cross something off my list and feel a motivation boost.
• If I just did something that wasn't even on my list, I write it on the list and I immediately cross it off it. It tricks me into feeling like I did a lot, and it works even though I'm aware it's a trick.

Good luck on the journey, I wish you the best!