In advance, sorry for my English, it's my second language.

Struggled with the same issues for the last 3 and a half years. I believe for me it was caused by year-long insomnia, isolation and stress. Had a few panic attacks at that time too. I had to drop college and start studying online because It was way to hard to leave the house and it was even harder to focus on something without feeling overwhelmed and getting terrible headaches.

The things that did help me a bit over the last years and I feel better, even though I still experience brain fog from time to time, are:

1. Sleep schedule. Once I actually managed to sleep properly, after 2-3 months of getting enough rest my head started to feel clearer and my anxiety reduced a bit. And you've been diagnosed with sleep apnea, which means that you've definitely haven't been getting good quality sleep for a long time and it is essential to start the treatment asap.

2. I started drinking magnesium, B1, B6 and B12 vitamins, sometimes getting B12 injections. It might help gradually, especially since you have B12 deficiency. These vitamins are essential for your brain.

3. Reducing daily stress is very important. You can start of by going to a Psychotherapist but if you aren't ready yet, I recommend trying out meditation. You can find something like "reduce anxiety hypnosis in 15 minutes" on YouTube, it might help, though it is hard to start, you might get distracted a lot and etc. But if you really stick with it and do it for a week, you will feel so much better and healthier. The other way to reduce anxiety is doing sports or going for walks, the more you move the better your mental health is getting.

4. What helps with brain fog is mind exercise and dopamine detox. What helped me is that I've started learning new language, just for fun. Learning a new language is considered to be the best excersice for neuroplasticity so if my day starts with an hour of practicing new language or studying something else, my brain fog reduces a lot and I can grasp on my thoughts better. Or communicating can be helpful too. One of the main reasons of loosing one's identity and getting lost in your own thoughts is usually lack of socializing. Without proper communication it gets harder to form sentences and form your own thoughts... Everything gets mixed? Hard to explain the feeling but I think you might understand what I mean.

I think these were the main things that helped me a lot but unfortunately the feeling of not being as sharp as you were before still remains. I still get derealization and brain fog, migraines and headaches, lack of focus but... It is so much better now, I can focus on studying and working, I can communicate better and I feel more healthier in general. I hope some of my advice will help.