Inability to concentrate can be a symptom of mood disorders (caused or aggravated by hypoglycemia). Following a hypoglycemic diet and supporting the adrenal glands can help to correct this kind of imbalance. There are also a number of herbs that help to improve concentration, memory and other brain functions. These include ginkgo biloba, Siberian ginseng, ashwagandha, gotu kola, and bilberry.
Example of a brain support formula: https://vitamost.club/shop/brn/