@Olga
The cognitive assessments you had sound particularly exhausting. The other part of the exhaustion is the fear you have about the outcome.
I also had a battery of cognitive assessments done on me as part of a fitness to work assessment. My employers were trying to prove that because of my disability I was unable to do my job. I was very anxious about the testing and found it really challenging to do my best for the 5hrs of testing. I performed really well on the tests, but they still highlighted all the areas that I was weaker with - suggesting there was a problem with my average scores as they were below my other scores. Another test I had right at the end of the day, I guessed a lot as I was too tired and just wanted to go home.
Try to stay positive about the testing. Have you been tested previously for them to compare your current results to the past? Everyone has areas of weaknesses and that is ok. If they do believe that you are having cognitive decline then ask what you can do about it. Possibly changes to medications, or mental excercises such as crosswords.
I did see a show on SBS called How 'mad' are you? It included a participant Cameron who's mental illness is often associated with cognitive decline, but he had made efforts to offset this and did really well. Worth watching if you can.
Best of luck.