For many people, feelings of abandonment begin in childhood, when they are most vulnerable and aren’t receiving the emotional or physical protection they need. Their environment is unstable, and the feeling of "I'm alone in this world" or "the people who should love me don't care" are internalized.
Feelings like that don't just go away as you grow up. They fester and cause a lot of pain, often turning into a destructive force as a teen or adult.
But it doesn't have to be that way. You can heal, and you can be happy.