As a sufferer of adult ADHD and already tried multiple nootropics even including illegal drugs. I'll suggest to start with the basic 1st for study concentration and memory.

1st have to look at lifestyle and foods that you take.
-Nothing beats a Good morning exercise + sunlight. It help with concentration and mood for the whole day.
-try your best to eat healthily with variety. Plenty of vegetables and healthy fats. Try your best to cut junks foods, sugars and carbohydrates.
-cut coffee because of tolerance and it increase cortisol/anxiety instead get a good green tea or chocolate drink(dark cocoa powder and make sure it is not Dutch processed).
-learn to be organised in life and in study.

If all that already implemented, start with basic supplements
For study I'll suggest
-magnesium
-vitamin B complex
-vitamin C
-grape seed extract
-omega 3 (suggest from krill or very good quality fish oil(please keep fish oil in fridge because it easily rancid)