Dude you don't need anything external.
Just doing simple maths instead of using your calculator makes your memory better, and also sudoku is supposed to be very good, but trust me doing simple grocery maths witll get you where you need to be with memory after like a few years.
However Gingko Biloba is supposed to be very good for memory.
If you have a well balanced diet, then you won't need those omega-3's or multi vits - as long as you eat enough veg, nuts, seeds, and fish - then you will get all the vitamins you need (hemp seeds for the omegas, to supplement the fish).
No supplements are needed unless your body starts exhibiting external signs like mg/calcium spots on your fingernails.
Or you cut meat, eggs and dairy out of your diet - you will probably need supplements then for a while, until you work out dietary alternatives
What colour is your urine these days? Fluoresecent yellow I'll bet - excessive vitamins/minerals being excreted.
As for focus, concentration - well, if you have not practiced these things, they will not be easily achievable.
Use "candle gazing" - every night at dusk, take out a candle, sit with it burning at eye level, stare at the flame, and relax your gaze. Do this for ten minutes every night, and every time you need to blink, close your eyes instead and focus on the shape of the flame burned into your vision - the green spotty image or whatever colour you get - until it disapears into the blackness and then open your eyes and continue.