show me the meaning of being lonely

LOnely,
is this the feeling i need to walk with forever?

you've become really important to me,
almost unnervingly so.
i know you are there,
always....
but you are still missing in my heart,
Tell me why I can't be there where you are.
paradox....

i cant really explain it all through words,
when we went out for yumming last night,
you are there beside me,
but,
suddenly i felt the loneliness again,
the insecurity is drowning me,
the darkness is consuming me again.

i know its my problem all along,
i went depressed at the most unexpected time,
with no particular reason at all.
I'm very rarely alone in the sense that another person's physical presence is usually with me.
Then again, sometimes no matter where I go or who I'm with,
I'm alone.
I can be in a room with a hundred people and still I'm completely alone.
i dunno why,
i just can't control it,
its just me,
maybe its something i had to live with for the rest of my life.

PLease,
show me the meaning of being lonely.