It's usually male birds who are more interested in mirrors but there are some females that like them too. To use a mirror as bait or a reward, you need to take the mirror out of the cage at least temporarily (assuming that there's one in there). As long as she has a mirror friend inside the cage she won't have any motivation to come out. You can open the cage door and put the mirror just outside the door, trying to get her to poke her head out a little bit to kiss the mirror. Once she's comfortable doing that you can move the mirror a little further out, and keep doing this until she's all the way out. You need to have a plan for getting her back in the cage though. Are her wing feathers clipped or is she fully flighted?
You can also hold the mirror in your hand for her to look at whether she's inside the cage or outside. This will help her get used to your hands and the idea that your hands deliver nice things to her.
Instead of mixing the millet bites in with her food, you can use them for food bribery. She'll still get to eat them but they'll be a special treat that comes from you instead of something she can get by herself any time she wants.
Most budgies like to eat leafy greens, although you might have to teach her to eat them. Try clipping them to the cage bars in an easy to reach place instead of putting them in a food bowl - foraging style food is more interesting than food in a dish.
If she's only eating seeds right now, that's an unbalanced diet that will cause health problems over time. But there are lots of good foods that she can learn to eat, including vegetables and pellets. If you're in the US or Canada you can easily get Nutriberries, which are nutritionally equivalent to pellets but they look like seed balls, and most birds love them.
Here's an article on the best diet for cockatiels:
http://www.littlefeatheredbuddies.com/i ... ldiet.html But it applies to budgies too - both species come from the same place and have similar diets. The main difference between them is that cockatiels can eat some seeds that are too big for budgies, like sunflower seeds. But a budgie could probably manage a sunflower seed that was already out of the shell.
Here's an article with tips on teaching your bird to eat new foods:
http://www.littlefeatheredbuddies.com/i ... rsion.html It can take a long time for them to accept a new food so don't give up if she doesn't go for it the first time. A better diet will help her live a longer, healthier life. Your bird is still young so she might accept new foods more easily than an older bird would.