Can’t focus

I dont get how people can go to classes and really focus on it, and yes I do like what I am studying but I just cant focus, and its so annoying.
When Im in class all I can think about is really nothing and even tho this might sound weird, its true, Im actually blank at class and sometimes these thoughts of hating myself past trough, or me remembering stupid things instead of listening.
Even sometimes when my friends are talking to me I just cant really hear them, because my mind literary goes somewhere else, and dont ask me where because I dont even know where it goes.
I been so frustatred by this lately because it had become worse and its really so hard for me to focus and Im tired, because also my grades are going down, and I get really sad by that and Im just sooo tired I dont know what to do..