I hardly understand what I am reading and easily forget. I also feel tired when reading. What can I do?

It is very common to not understand what you are reading. Give the reading your undivided attention. When you read concepts over and over again, it will enable you get detailed information you need. Write notes in your own words to clarify your understanding. Read a lot to improve your reading skills, including fiction which will usually reassure you that you have no problems understanding, it's more about the difficulty level in textbooks and scientific papers. Have breaks during reading, to give your eyes and mind a rest. Learn to evaluate yourself after every reading, to go over what you did and didn't understand. Most of all, don't feel you have to understand things immediately -- for most people that is unrealistic and it is in the struggle to grasp the meaning that real gains are made and even new insights.