I think a lot depends upon the way in which they let you down. If they do something immoral or unethical, that is very hard to get past. If they disappoint you by not living up to your expectations, i.e., they are not THERE for you when you think they ought to be, that is something you have to examine inside yourself, to see if maybe your expectations are unrealistic.
People, including heroes, are human. We all err.