Teenagers often may not like some kinds of studying because it seems they have no interest in some subjects. It's similar to our emotions can be weakened or lost by outside forces such as being overwhelmed, demotivated, or distracted. Forgetting what you have learned can indicate that you are stressed or overwhelmed with many things in your head. You need to find time to unwind, practice self-discipline, and communicate your needs to someone if this happens to you. Appreciate that being involved in your studies is important but take time to relax and take care of yourself by knowing that it's okay to need help from others whenever you are in trouble.