Upload a youtube link if you can. I like ones that are strictly violins or the like.
What are some good, sad songs on violin?
What Girls Said 4
Requiem for a dream (saddest I know of)0
My Immortal by Evanescence
Irene Adler's theme.0
What Guys Said 5
For strings I like Dallas String Quartet or Vitamin String Quartet.
Here's Meditation de Thaïs by Jules Massenet, with Sarah Chang as the soloist. I like this version.
Souliko is good.0
I can't help you with actual songs, but i can offer general music knowledge
Songs in a minor key always sound sad. Songs in a major key tend to be happy0