1) get pleanty of sleep. lack of sleep makes you forget things and can't concentrate.
2) read multiple times like someone said up there. hard work will get u to success.
3) what is it really bothering you? money, grades, love life, sex life, try to get a grip on the real issues rather than dealing with the symptoms. 
4) try mindfulness meditation at least 10-15 mins a day, look up online how to do it..will increase your concentration and you worry less..