I seen on a tv show once that a mother was having the same problem as you ane her kids were older. So what she done was she started to pure' vegetables and fruits and mix them in with things like cakes and mixed them in with meals the kids already liked. So for example if your daughter likes spaghetti or mac and cheese try puting some pureed vegetables in with it. My son is the same but hes only 2 he used to love fruit and veg but one of my stupid friends introduced him to lollies and chocolate and now thats all he wants but i dont give it to him. I have been doing the same as you tho giving the most healthiest option of the foods he does like...hope this is of some sort of help, sorry if it wasnt. Good luck! And remember you are NOT a bad mother by the sounds of it you are doing a great job =)