Have I seen a Dementor? No.
Have I felt a Dementor? Yes.
How, you ask?

You know there's a Dementor when you are sitting in a room full of people and tears are flowing down your cheeks despite trying to fight them back.They ask why?You mumble something indistinctly because you do not know why.

You know there's a Dementor when you are too exhausted to leave the bed after sleeping for 10 hours straight as if you cannot drag yourself through another day and again, you know not why.

You know there's a Dementor when you get down from a bus and suddenly in the middle of a conversation with a friend on a beautiful autumn afternoon you want to scream your lungs out and run away from the invisible and yet constantly looming chaos.

You see the sun shining bright and birds chirping alright but day after day you can feel it feeding on your soul. You can feel it running through your veins, poisoning its way through like the heaviest nothingness of all centuries. You can feel its nasty hunger for power over you as it strikes right at the roots that exist in the deepest parts of your heart and makes it hollow.

And what do you do?

There is no Patronus and yet you fight. You fight the battle every single day. You live with it and the struggle becomes a part of who you are and the only thing that remains alive is a tiny shimmer of hope that maybe one day you'll stumble upon your Patronus and all of this will be over as if it were a bad dream.