They really are good.
Don't you doubt it: I will correct you.
I want you to learn to speak Spanish well after all.
But that doesn't mean we need to be so serious all the time.
figure out why you made the mistake
see ways to avoid it in the future
listen to suggestions
try some exercises
leave the issue on the back burner for a while
come back later and try it one more time
Mistakes are good because they make us aware of the difference between languages;
they make us aware of the (in)consistency, arbitrariness and, sometimes, clarity
, of their grammatical "rules".
(They are not rules really; just approximations.)
They help us to better understand how real people really use their language... and how we use ours as well.
They help us to learn how we can use our own language to learn the new one... and how we shouldn't.
Take a look at the courses
I'm offering this season
(Fall 2, 2017: November 2nd - December 21st).