Abandoned

Being the only one left out is not fun
Being the only one that's left behind is not fun, and scary

but what makes one feels abandoned, I wonder? Especially, one is surrounded by people and things in general?

The equation to the feeling of abandonment =
1) you have to have this strong sense of attachment to something/someone. Doesn't have to be mutual. As long as one of the party is feeling the "attachment" - it's done!

2) the need to part/detached or lost of connection. That's the point where the sense and feeling of being abandoned come about.