OP, Let us see the positives. you are able to focus for 15 minutes. Its a normal thing to loose focus when we are involved in so many things or tired. If there is a dead line, we burn midnight oil?, Why? because we know the consequences. Just imagine the consequences, that may give some motivation. Check my old post Unable To Concetrate And Stick To Goals, It may give some idea. What you need is determination and drive to study. In my job, I have to study different innovative stuff every day, and have to be an expert in that topic to publish. It takes lot of time, effort and creative thinking. I spend a part of the time, to collect materials and read. But I think my subconscious mind do the job. But, what we need is persistence and drive to stick to our goal. Find the time you are very focused (early morning or night) , avoid all distractions (keep your mobile away), focus 15 minute, after that just think what you have learned. In your notebook, write down what you have done for the day. Next, day try for 20 minutes. Usually the time to concentrate in one stretch is less than 40 minute, we need a break. In my case I can focus very well for hours if there is a deadline. Nothing bothers me. Not every day. We need to motivate ourselves for that. Check the tips the link I have shared. Also, you need good sleep, healthy mind and body. I also take thyroid medication, so don't worry much about your thyroid problem. I usually play music ( instrument , mostly flute, meditation music) in the background, that relaxes me and help me to focus. Some people use head phones to avoid distraction. If anything distract you force your mind to do it and sit there (don't get up) till you finish the block you wanted to do that day. Once you are done , just check what you need to do next day and exit. Next day start again. That way, your mind will plan things and subconsciously focus on it. If you can study 15 minute, use that kind of blocks every day. Take baby steps and set up realistic goals. One day you will be amazed on how much you have learned. If its a difficult thing to do, do this as the first thing to do every day. Working long-hours wont make anyone productive, we need smart work, you need to assess what should be done for your certification. Make a list of things, study one by one day by day. You will reach your destination. Never ever give up.