Thanks for your query at Doctorspring.
I have gone through your query in detail and can understand what you are going through.
Like intelligence and memory, capacity to focus (concentration) varies from person to person. You have not stated that it a change from your previous ability to focus and I assume that it has persisted all your life.
Further certain factors do change your ability to focus (concentrate)
1) Sleep disturbance
2) Depression anxiety
3) Environmental factors like comfort, sounds.
4) Interest in the activity that you are trying to focus
5) As you have mentioned you snore then even Obstructive sleep apnoea.
6) Anaemia (low Hb in blood)
You need to see a psychiatrist who can evaluate you for the same. You need to undergo a blood test (CBC, TSH), sleep apnoea study, mental state examination and some other tests as required. Unfortunately EEG would never have helped you.
Hope I am able to answer your concerns.
If you have any further query, I would be glad to help you.
Wish you good health,
Dr. Srikanth Reddy M.D.