Thanks for providing more information.
Firstly your symptoms can be associated with depression and anxiety issues.
And you can ask your doctor for some psychostimulant to add with sertraline.
Even your blood pressure,blood sugar levels should be checked.
Low blood pressure,low blood glucose levels can result in such symptoms.
And further blood work like CBC,ESR should be done.
You should start yoga,deep breathing exercises,meditation.
Nutritional Deficiencies may be the most common cause for lack of concentration.
Start a multivitamin supplement.
Psychotherapy,cognitive behavioural therapy can be advised.
Here are some supplements that can help concentration:
-DHA and EPA (both found in fish oils)
-Milk Thistle Extract
I advise to consult your doctor for further management.
Hope this helps.
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