To think that something is smaller than it actually is
To overrate is:
To overload is:
To undermine is:
To do less well than you could do,e.g.,at school.
To do something too much
To underestimate is:
To give someone too much of something
To say that something or someone is better than actually is
To gradually make something weaker or someone less confident
To underachieve is:
To overdo is:
