There are two possibilities. One is that I am entirely unique - not a person in the world is like me, no one can think my thoughts. I am a unique contributor to the world, but at the same time, no one can really understand me. Empathy is possible, but true sympathy is not. For all my communication, all I'm doing is referencing things other people have felt and thought. We're all different - and because of that, we're all utterly alone.
The other is that I'm really not that special. Somewhere out there is my doppelganger - my many doppelgangers, likely. All my ideas have been thought before, some executed, some rejected. We may as well all be one - we're all so similar that everything that is not exactly alike is purely cosmetic. All that's required to make another me is someone with a superficially similar facial geography and a new wardrobe. I have absolutely no hope of being special.
Or worse - everyone IS the same, except for me. I am perpetually and irreparably locked from most shared human contact. I am unique, and because of that, I am a monster. I was never meant for this world.
Guess what my recurring psychiatric ailment is? If you said "severe depression", you win a prize! (the prize is whining)