Hello Hayley
Not sure I can be of any help, but DD (6) sounds very similar to your DD. I'd also describe her as very sensitive.
She seems to have a pattern of being more 'normal', followed by a terrible patch where everything affects her deeply.
She's started to have friendship trouble (one of my biggest fears, and not unexpected), and she constantly talks about her friends 'being mean', 'hating her/each other', or feeling sometimes stuck between her two best friends. Tears are a daily thing after school at the moment. Like your DD she really thinks everybody should get along and just cannot process unkindness.
I spoke to her teacher, only to be sure she's not the same at school (she seems to cry only at home, but she has always been generally 'whiny' and cries when she hurts herself longer and harder than other kids). Her teacher says she's been fine, but she has noticed little tiffs in some groups of girls, so they are planning team building activities, and generally encourage them to play with other people and be nice to everyone.
Like you, I also worry that she will be left out because of her personality. My approach is to gently encourage her to get over her upsets, to realise that people don't always mean what they say, and that you cannot be best buddies with everyone, no matter how great that would be. I try to talk to her at bedtime everyday, gently, without insisting as that seems to make things worse.
We have a book called "How to be a friend", but we only just started reading it. She seems to be taking some of it in, and I think I'll buy her one called 'What do you do when you worry too much'.
Ultimately, I think it helps both DD and me just to know that there are other children (and parents!) in the same boat
. In time, with our support and as they grow up, relationships will become easier - I hope.