"Get something off your mind" refers to something bothering you and by verbalising it(telling somone) you take the pressure off of yourself. An example could be that someone spoke to you in a way you did not like to you have a word with them about it. "Get your mind off something" means that you cannot stop thinking about something. The thought keeps reoccurring in your mind. It could be something that happened or something said to you. An example could be a horrible image you cannot forget.