You should remember that you’re the adult, you make the food: you have final say.  Stop giving the mac n cheese and grilled cheese and only give her something a bit better.  Don’t make a separate dinner for her.  Give her what you eat— and eat with her.  Children learn by example. 

Don’t mention vegetables like they’re a punishment.  Think of some fun things to do while eating them or say her favorite character loves a particular vegetable.  We made up the “green bean dance” years ago and my kids still do a dance when eating haricot verte or string beans.  We said that mushrooms were the Power Rangers favorite food when my eldest was 4 and he suddenly didn’t want to eat them, but then he loved them again because he loved the Power Rangers.

My middle son (6 years old) still throws little tantrums some times and I just ignore them.  It does no one any good by giving in.  He’d just learn that it would get him what he wanted by bad behavior and I’d never get him to eat well.  He almost always finishes his meal and usually likes it too.  He will get dessert as a reward for eating well.  But no dessert if he hasn’t completed as much as I’d like (not necessarily all, but most always at least the veggies).

Also try stepping back, not pushing but having veggies there and eventually she might be willing to try without a fight. 

If it persists… choose the best mac n cheese (no yellow 5, no hormones) and put veggies in soups, smoothies and pastas until it passes.

Let me know if any of this helps!  Good luck!