One of my disabilities is pretty obvious. Since I'm in a wheelchair, people rarely ask me why I need accommodations for things. Sure, I get asked why I'm in a wheelchair, how long I've been this way, and so on, but no one has ever questioned why I need a wheelchair or if my disability even exists. For those with non-visible disabilities, the opposite is often true. While they often can "pass" as able-bodied in a way people like me cannot, they're frequently subject to scrutiny or disbelief if they reveal their disability to others. Although non-visible disabilities can be physical, they can also be mental (as in mental illness) or cognitive (meaning intellectually impaired.)
So, even though it's easy to see the cerebral palsy, it's not so easy to see that I also have depression and anxiety. Now, once I actually started telling people, which took several years (thanks, societal stigma), most people were super accepting and understanding. However, there were, and still are, people who just didn't get it. They looked at me with confusion: "But how? You seem so happy!"
I can understand how it would look that way. I have an excellent GPA, am very involved in my sorority, and have a few other extracurricular activities. I have plenty of friends and am generally pretty happy. But that doesn't mean my mental illnesses aren't real. I have what they call "high-functioning" depression and anxiety. For the longest time, I never considered my mental illnesses "disabilities," because, well, most of the time they're not disabling. I can get through my life fairly easily, with only a few setbacks. For example, negative self-talk that makes me think everyone hates me, and panic attacks that occur whenever I'm in crowds or need to give a presentation. Even then, my anxiety sometimes doesn't present externally. I might feel like my heart is going to explode and every cell in my body is screaming at me to run away, but at least I don't "seem nervous" to anyone else.
Like many non-visible disabilities, "high-functioning" mental illness is both a blessing and a curse. On the one hand, I'm perceived by society as able-minded, which means I'm not subjected to frightened glances, discrimination, and so forth. On the other hand, I have to work a lot harder to have my illnesses validated by others. One of the reasons therapists say that "high-functioning" anxiety and depression can be difficult to diagnose is exactly because it's not as visible. Because we can go about life as fairly "normal," our loved ones (and ourselves) can trick us into believing that we're better off than we actually are. Telling us that these feelings are "all in our head" is about as useful as telling someone with asthma that it's "all in their lungs." Yes, it is. THAT'S THE WHOLE PROBLEM!
So, whenever someone tells you that they have a mental illness, but it doesn't "look" like they do, here's a tip: believe them anyway. Chances are that they know themselves a lot better than you do. Ask them what their triggers are and what their symptoms are like. Anxiety, depression, and other mental illnesses don't manifest themselves in the same way for everyone; people need or want help in different ways. Sometimes, just being there is enough.