Sounds like how i used to be, i couldn't focus to save my live. i recently saw a psychiatrist and he prescribed medicine to help with concentration - it definitely helps but the side effects are awful so if you can get into a routine without needing medication i think that is the way to go.
when you have ADD-like symptoms, you have to be very disciplined. here are some of my tips i use:
1. seclude yourself. you have to remove all distractions. either go to a library or find a desk/table in a quiet secluded place away from people. do not bring your cell phone. try to avoid using your computer unless doing online practice problems. GMAT studying requires a lot of concentration and any distractions can completely throw you off.
2. make a plan. this is what i find most helpful. i have trouble organizing my thoughts in my head, so i make a list to plan my day of studying, and i stick to it. for example, i will set specific goals such as "do 30 problem solving questions", "take notes on chapter 5", etc. i feel guilty when i can't check something off of my to-do list at the end of the day.
3. take notes. when reading, it is easy to let your mind wander and suddenly you are not retaining what your eyes are reading. if you take notes while reading, it will help to keep you on task and will help your brain retain the information.
4. eat healthy, get enough sleep, exercise. a healthy mind requires a healthy body. healthy food will give your body the fuel it needs to keep you focused and feeling good. exercise works wonders for thought clarity.
5. tape a photo of your dream school logo near your study station. when you find yourself getting distracted, look at the logo and remind yourself of why you have to do this. a little motivation can make a world of difference!