I'm in agreement; she shouldn't pretend to be a flowergirl in any way.
She's also showing you a vulnerability or weakness she has--that's great that you're getting this evidence; now you need to help her.
I spent a lot of time instructing my kids on what to expect in situations, and I also "pronounced" things like "I know you want to feel special, but this is not acceptable. It's not your turn. It's someone else's turn. You would feel upset if someone else took your turn, right?"
I think those lessons start with things like trucks and swings, etc.
If you're feeling at a loss for how to coach her through this, please consider reaching out for some coaching from a child psychologist or family counselor. They are pros, and they will have all kinds of tactics and suggestions and things they can teach you so you can be your girl's most powerful coach.
It's not a kindness to kids (to anyone) to indulge them in order to shield them from disappointment and pain. Better by far to have them experience that feeling of rejection, that feeling of being left out, while they are young and the stakes are small.
That's an important life skill, and only she can teach it to herself (you can model proper behavior, and you can insist on creating situations in which she can learn it, and you can provide motivation for her to learn it, like negative consequences if she exhibits behavior that's socially unacceptable).
But it's crucial for her! It's far more important that she learn how to NOT be the center of attention, and how to focus on other people, than it is that she not be sad or throw a tantrum when she's unhappy.
Good luck!