Like muscles that are overtired and can't work properly any more, good mental habits can be undone by mental stress or overwork. Those "good mental habits" might be anything you've learned in your lifetime, such as "not having panic dreams about being naked in school just before a test you haven't studied for" or "not slipping into bottomless depression for no reason".
Mokalus of Borg
PS - The funny thing is, sometimes you don't know you have these good mental habits until they disappear.
PPS - And then you don't know how to get them back.