What Causes Lack Of Concentration With Wandering Thoughts?

Hello. I am an 18 year old college student. Since about sophomore year of high school, I have begun to notice that I am having difficulty paying attention. I will be sitting in class, or reading a book for a class, and my mind will wander between various unrelated thoughts. Ive found it very difficult to study due to this. I am on the pre-med track with a computer science major. I just don t know what to do to make my focus better. I have tried meditation, closing all electronics, but I keep getting lost in my thoughts.