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