First I would like to appreciate your courageous move and determination. As an elder I'm delighted to give advice to a dedicated youngman like you...so what you have to do is building a solid foundation, because that's what actually differenciate a high performance student from the average. First I want you to look through all your subject matter and find areas that you have a serious background problem(like concepts you don't even know before...but it is thought as if you know it's preliminary). Once you find out all of them...go back as far as a point where things start to make sense( it's completely okay if its far down to 7th or even 6th grade textbook) then you start to build you foundation from the bottom up. You can't rush it man take care on this one. People may say you don't have time to go back but trust me that's the only way you could make this work. Like trees have to grow their roots before they even grow those huge shoots. And we this process I bet you will find out why high performance students enjoy working on complex problems and don't get tired as much. Through time you will get the satisfaction of learning to know the concepts and not to get tested. You may get exhausted at first but when days go by...you will find your self studying and forget to even check time. That's when you know you're actually making progress. May god help in your journey and I wish you a bright future.