I have always had a hard time concentraiting, what is a common reason for this in young adults and a good remidy?

Unfocused. Common reasons for problems concentrating include: poor sleep, lack of exercise, inadequate nutrition, supplement/vitamin misadventures, substance abuse, caffeine/energy drinks, stress, depression, and anxiety. Underlying medical disorders can contribute. Add can be meaningfully evaluated via self-report scales/psychoeducational tests. Excellent prognosis with stimulants + metacognitive therapy.
Not specific enough. One needs to know a lot more to make a diagnosis or suggestions for remedies. See a doctor for more specific help.