I am studying at college and I always have problems staying focused and have very poor memory, which seems to have deteriorated since I left school.

Poor focus deteriora. If the onset was present before 7 years of age, then it is possibly adhd, but with your indication that the deterioration is more recent, your memory needs to be evaluated, and see your doctor. A common cause of slight memory loss, and poor focus with onset in your 20's is possibly depression or anxiety also.