What Mental Disorder Causes A Lack Of Satisfaction After Understanding Something, With Persistent Thoughts That I Haven T Understood It?

What is the mental disorder i am suffering from,I am a student ,earlier whenever i used to read somenthing or understand i used to get a satisfaction but from last 8 to 9 months whats happening with me is after understanding something i dont get the satisfaction and my subconsious mind keeps thinking that i have not understood the thing,please help me ,this thing is killing me?