It's much easier to sympathize with people with physical ailments because, for the most part, you can see it. Mental illness is an invisible illness, and not one that people are generally very educated about...even when they know people (eg. their own children) are dealing with it. The only thing you can do is do your best to educate your mom and hope that eventually it clicks.

It is also possible that she is deep in denial because it is extremely difficult to accept the fact that mental illness is a lifelong struggle that there is no cure for.