Google - I am having difficulty with focus and concentration. For decades of my youth I heavily used

First off, we are not Google so if you want to ask them you need to contact them directly. We are question.com.

Secondly, you really need to have a complete physical and be honest with your doctor about why. As rough as it sounds, certain drugs can severely damage your ability to remember and or concentrate. Some times for a long period of time. You need to find out if this true for you and if so what the professional medical people suggest you do to remedy the situation. That may or may not involve vitamin supplements. They may help or do nothing because something else works better.

Best wishes for a better future!

Many drugs destroy B vitamins. The fix for that is easy and cheap: B-50 or B-100 pills or nutritional yeast. Nutritional yeast tends to have variable strength, and the more you need it the more likely it is to cause gas. Vitamin B2 is a water soluble dye that turns urine bright yellow. When the color fades, it's time for another dose.

Read some books about nutrition so you know how to eat right. These books are fairly old, but still the most popular introduction to the field.
www.amazon.com/Adelle-Davis/e/B001J3RULO
www.adelledavis.org/
cronometer.com/ (I haven't tried the software.)

Doctors have no training in nutrition, only drugs and surgery. A doctor recently challenged me on that, saying he was a registered dietitian. He didn't even know the difference between nutrition and dietetics. (Dietetics is the study of what foods should be swallowed. Nutrition is the study of what happens after they are swallowed.)