I feel that way when I'm having a depressive episode. It becomes very hard to concentrate or focus. If that's the case for you, it would be worth seeing a psychiatrist. Medication addresses the depression, and then I can concentrate again.

If you have female hormones and are going through menopause, that could be a factor as well. I would consult a gynecologist about it.

If you are over 60, it might be dementia. In that case, I would see a doctor who specializes in diseases like Alzheimer's.