Don't wanna offend you but she really has a few Mary-Sue traits, mostly from the "Tragic Past" area. You don't need to make an OCs past THAT tragic, regardless to her name. Simply one or two heartbreaks would be enough for her. I saw you got plenty of advices down there and I say they're really good. Give a few or even one thing that can make her happy and let her depression go. Give her life a meaning.
You know, I also have an OC similar to yours (she comes with her own story). Her name is Sonata, a sorrowful little filly. Her parents divorced and because of that her mother comited suicide. She moved to her grandmother where she lives right now. She goes to the school too, where Tiara and Spoon makes fun of her blank flank as well but they shut up as she repiles and counters them on her own way.
She finds her happiness in making "music" by hitting different stuff (closing windows, cling glasses, shut the door, etc...) in the classroom to rythm, making her earn her cutie mark that she begins to hate thinking it's meaningless. Because of that she tears it apart with a knife.
Sonata is a complete deviant, with an "I don't give a sh*t" attitude. She doesn't want to meet others and throws the word "death" around like it's a child's toy (e.g.: when once she comes to school with a rope around her neck and the fillies look at her with eyes wide open she simply asks "Like my new necklace?", or when Cheerilee warns her that she'll contact her mother of her behaviour, but Sonata simply "gives her her adress" (which will lead Cheerilee to the Ponyville graveyard)).
So she goes to holidays instead of work? Make sure to have that thing she likes a drawback, consequences, an obstacle. Sonata like to play on not-instruments but she got a godawful cutie mark for that she had to slice into pieces. See? The benefit and it's price?
She has more things to like, but sometimes when the love-like holidays like Valentine's Day come up she just can't stand it. I guess I can see what you mean, although I'd have to fix alot of things. I can say she still has a family, too.