This feeling

Toska- noun  - Russian word roughly translated as sadness, melancholia, lugubriousness.

 "No single word in English renders all the shades of toska. At its deepest and most painful it is a sensation of great spiritual anguish, often without any specific cause. At less morbid levels it is a dull ache of the soul, a longing with nothing to long for, a sick pining, a vague restlessness, mental throes, yearning, in particular cases it may be the desire for somebody of something specific, nostalgia, love sickness. At the lowest level it grades into ennui, boredom."

--Vladimir Nabokov