Grass is always greener...
The deepest despair of my mental issues is worse than my physical issues, I'll give you that... but...
You think you will be understood if you have a physical illness. I have one or two or three depending on how you see things. To be understood for your physical illness it must show. People must also have knowledge of it. People also must have felt fear of the illness or felt something similar so they can relate.
Many illnesses don't show on the outside. Many illnesses people have not even heard about. Also, people think if you have a physical illness, take pills and feel better.
My illness don't show. It has a stupid name (autoimmune overlap syndrome with lupus traits). It can probably kill me but if I say so, people laugh. "Oh you who look so healthy!" Treatment is worse than the illness. When I'm in pain it doesn't show. When I feel like I'm going to pass out I look normal. If I ask to sit down people look at me like if I'm dumb. If I can't keep up with someone, I should "exercise more and stop being lazy".
Believe me. You don't want this.
I know exactly how you think though. I also wish people had more knowledge and understanding and compassion about mental illness instead of being judging, callous jerks.

But maybe... it's just how THEY are, no matter what is wrong with us. It's something wrong with THEM.