Wanting to achieve a good level of concentration, you might find a serious difficult duty to absolve, an urgent assignment, and thus you might start working at it, deeply absorbed in a diligent and even perspiring exertion if your work is more material.
Whenever you are working at something, but that something only receives a 5 per cent of your attention, then that something is definitely not interesting for you.
On your own, and also in bright interaction with colleague or superior or mentor, you might agree on a more serious or even severe control of your work and of your attention.
You may be unable to remember things because such things do not wake an ardent exclamation of ahaa!! within you. You may be unable to remember things, words, sentences, and more or less long numbers, because that of remembering such things may require a mindfulness and a particular training that you do not yet have.
I am neither good at remembering numbers and sentences, but a way of remembering numbers and or sentences will be to reflect them within your own mind and to repeat them mentally and or vocally. For instance after having read a sentence you might try and see if you can remember that sentence, and you might exercise in remembering it, until you remember it completely and fluently. You will realize that some sentences are sort of automatic, and even glowing, for you to remember, while others are annoyingly nearly impracticable. You might try and more diligently exercise at re-reading and remembering those sentences that you cannot immediately remember.
A way to better concentration may be to also try and reflectively learn some poem and some prayer, or even a song, by heart.
If you will learn such things by heart, and will like to repeat them, and while repeating them you will meditatively reflect upon them, upon their meaning and upon those associations that they may attract you to,
you will be able to see that you have reached a good level of possibility of concentration, a level that is transferable and that will make you more proud.
Along with all this, it is important to have the habits and to do the things that will corroborate your health. You might see if you normally eat good health-giving food, and if you retire for the night never much later than midnight and resting in your own bed until six or seven in the morning.
You might also expose yourself more to fresh air, while also doing some breathing exercises.
Then you will make your own evaluation about your strengthened ability to concentrate and so see that you can be glowingly proud of it.
Have a laugh. Good luck.