There's a mental condition called 'depersonalisation' that can be, for many, very distressing. It's a failure of the mind to correctly deal with its own existence, leading to feelings of unfamiliarity about the familiar. I'm going to have a go at describing what it is like, as I'm a sufferer.
Imagine you are actively involved in social interactions, perhaps having conversations with friends in a bar. The atmosphere is happy. Suddenly, you feel a chill. A wave of anxiety passes through you. You start to sweat. At the same time, your perspective changes. You aren't in a bar having conversations, instead you are in a cinema, watching friends having conversation. They look at the camera and mention your name. You hear a voice respond. Your voice. Not wanting to appear deranged, you try and forget this new perspective, and the conversations seem to carry on. You calm down, but you can't get the feeling of distance out of your head. It's there, all the time, waiting to pounce. Later on you try and recall what happened. Your mind starts to whirl. You imagined yourself in a cinema. But what is the cinema? It's your own mind, of course. But if the cinema is your own mind, who is the person sitting in the cinema, watching your own life go by? It's your own mind, of course...
You realise you have been going through life surrounded by the strangeness of yourself. Reality is divided into two parts: the universe, the film running on the screen, and the you, the cinema, the projectionist, the director, the audience and one of the actors. You can never escape this feeling.
As a child, I found that I needed to start wearing glasses. Putting them on, I had a shock. I realised that the glasses were separating me from the universe.
This is not a state of delusion, not solipsism. You know what is real, and you know others are real. But the deep strangeness of your existence is there all your life. There can be long periods during which this strangeness can be accepted as normal, and there is no distress. But during times of tiredness, anxiety and depression it can come back with a vengeance.
It's a phobia of your own existence. It can't be run away from, it can't be avoided. It has to be tolerated, minute by minute, day by day, year after year.