I think what I would do is tell her she's allowed to be sad. It's the most normal and healthy emotion to feel when bad things have happened in our lives. And things have happened in her life that you wish hadn't happened because she is lovely and doesn't deserve them.
Then I'd say, when we are sad, it is incredibly important for us to take extra good care of ourselves, so the sadness doesn't take over our lives. It has a place, but so do more uplifting states of mind - like self care, cosiness, happiness, playfulness, hope, silliness, bravery etc. So when she is sad, she needs to gather together some things that help her feel secure and loved, by herself and by you, so she doesn't end up getting stuck inside that sadness feeling powerless and sadder than she needs to be. Maybe a favourite cuddly toy, a hug from you, a cosy blanket, a nice healthy snack, a soothing song, book, favourite happy cartoon or film, maybe a calming game to play - an easy card game, or lego.
Longer term, you could help her discover an activity she enjoys that puts her in a state of flow - where we are happily absorbed in what we are doing. Drawing or painting is good for this - making music, any craft activity she likes, maybe dance or sport or singing.