This quote from the anime shouldn't be a cause for fear, but for joy and celebration.
Imagine the erasure of all your self doubt, negative feelings and things holding you back, sure they make you "you" but they aren't worth holding onto, becoming one with everyone is an upgrade, you get to be something else, something far better. Something utterly wonderful.
This is why Shinji was wrong to reject instrumentality. He should have taken the plunge and accepted it, merged and become one with everyone, including those he loved. And his doubts and negative aspects of his self would be gone as he joined everyone in Lilith's egg.
Only a fool would reject such a literal heaven to go back to being an individual on a dead world.
How could you ever go back after experiencing instrumentality, you'd be a lost soul, disconnected from the spiritual umbilical cord of that true and pure existence. Nothing you could ever experience as an individual would ever compare. Ironically as an individual You'd be nothing.
But with instrumentality, you are everything. You are immortal, you are safe, you are forever.
What could possibly be better than a sweet existence of love and belonging and connection for all time?
Instrumentality was made for you.
Come out of the cold, return to mommys womb and be with your siblings for the first time.