Why does my mind keep drifting

I cant concentrate on ANYTHING! I drift off and daydream always!

I'm a 24 year old student and I have a hard time concentrating on ANYTHING. Not just school related but in life in general. I will try to read a chapter that is about 40 pages long. If I don't understand a specific paragraph I will not read the rest of the chapter. I just give up or lose interest. While reading I will often think of others things - (like what I saw on tv last night or what I'm going to do once Im done reading this) and I always forget everything that I've read and have to start all over. Its like I can say the words on the piece of paper but its not getting through to my head. I worry about EVERYTHING! Sometimes I can't even study for the test because I Fear Im going to fail it anyways so whats the purpose of studying? This is affecting me academically which is all that matter right now. Please advice what I should do. I've never had any mental issues and am generally healthy. I dont think its ADHD because I wake up every morning exhausted and am generally tired through out the day without doing anything