Lazy or is there something else?

I have a difficult time concentrating and always have. I'm fairly intelligent, but I always have trouble paying attention on a consistent basis. In high school, I would daydream, fraternize etc until the end of the class period but I always kept a B average so I thought it was normal. I am currently in college and have the same problem but my gpa is starting to slip. My major is psychology but I am also premed. I like my major and the material is not difficult but I cannot pay attention in either lecture or small classroom settings. In classes where the professor does not leave class notes online, or does not teach the material out of the book I am basically sol. All of my learning in college was done from teaching myself but this only takes me so far as some exams contain material that only popped up in lecture. I'll often wait until the last minute to study for exams. I do not do this on purpose but it seems like every time I sit down with a textbook when exams are not close I get distracted with other things. Even if I'm in a library I might sit down and try to read a textbook but end up daydreaming. I have goals and ambitions but my behavior is preventing me from making it. Don't get me wrong, I have tried paying attention in lecture and staying focused when reading but as soon as I let my guard down I am thinking of something else. I'm not saying its impossible but extremely hard. Although, I can pay attention when I am interested in the material. SO does it sound like I have some of ADD or do I have just a very type B personality? I've wanted to be a doctor since I was a little kid but my dream is slowly slipping away. I don't know how I can fix this problem on my own.