A feeling that is indescribable to those who have not felt it, and needs no description to those who have. That desolate feeling of being left out, left behind, and being all alone forever. This not only applies to girlfriends(though they bring the strongest effect) but also major life changes - graduation, deaths, etc... It is that hole that appears in your heart, which cannot be filled by your efforts alone. In such a case, crying occurs and is often encouraged as it heals the soul.
I want to be with her forever, but I know the day is coming where we'll never see each other again