What b@r!um asks is valid. I also have an anxiety/depressive disorder and I've gotten it under control so it doesn't interfere with normal grad stuff, but there is a lot in grad school that is panic-inducing. In fact, the experience (especially in a PhD program) is known for causing mental health problems in students that were heretofore healthy, much less exacerbating already existing ones. Some months have been straight up hell for me, and my illness is mild/moderate at best. I was shaking during my oral examinations, although my examiners couldn't tell. It's not that you can't succeed in grad school, but more so that you need to examine whether you have your illness under control enough that you can proceed forth.
You do not have to disclose your disorder. Schools are not supposed to discriminate, but the truth is disclosing a mental disorder still nets a stigma. When I wrote my statement, I didn't discuss my illness and just hoped that people would assume my bad semester was due to an illness (I think they did). If you choose to, you can just say that you were battling an illness throughout college that affected your performance. It's really none of their business what it is. If you have an upward trend in your grades, mention that you got it under control and point to your better performance in later years as evidence of that fact. But the problem is, this would be more acceptable if it affected one semester or even just one academic year - but it seems like it affected your entire program AND your GRE scores, so you may encounter problems.