Is it simply liting alone? Or is it longing for someone someting that you feel alone without?
Loneliness can be both above. However, like any other, feelings it can vary.
The most strong depressing and most dangerous loneliness is when your life among friends, family or the significant other, but they still can't or doesn't seem to understand you at all. So if you find that someone who understand you, and whom you understand, you will be the the foolest most idiot in this planet if you just let him/ her go.