What are good ways to study?

I study so hard yet continue to score low on me exams. I’ve tried a couple different studying methods and nothing seems to work. First I was memorizing the material and that did not help as I was memorizing but not understandings. The second time I red the entire nineteen chapters took detailed notes and asked my classmates to view theres as well. I watched YouTube videos to get a better understanding and to fully grasp the concept. I felt so confident on the exam and yet again I failed. In another class I used flash card to study and still did terrible. I really don’t know what to do. I’m stressing out as my grades are dropping fast as the semester is almost over. Can anyone give me tips? I am starting to feel like I’m dumb as I study so hard to fail and the rest of my class is doing great. I’m not going to give up but I do want to do better.