Hi!
I am a huge advocate for immersing the bird into your life as soon as it comes into the home for this reason. Often if we let issues like this simmer for months on end, the bird becomes comfortable how it is and doesn't want to change! Nevertheless, there is always a way to correct
Often you're going to want the cage in a common area, but for situations like this it may be best to move it into a different room (or go with the bird to a room away from the cage) for a while. This way you don't have to worry about the bird flying back to the cage, and this is the time you will want to work on the mindset of
good things come from me.
Have you tried looking away when you hand her a treat? Sometimes birds have a deer-in-headlights moment unless you look away. If she is still too frightened, drop the treat in front of her. Every time you walk by, drop a treat in her bowl/hand her one.
Another great thing that I always suggest is clicker training. The clicker is a powerful tool you can use to communicate with your bird. If your bird knows that when she comes out of the cage you may train her and she will get a treat, then she will be more apt to wanting to stay out in the future.
[ame="https://www.youtube.com/watch?v=jtzz8bdCpu8"]BTT Parrot Training 101 | The Basics of Training Your Parrot - YouTube[/ame]
Here's a video from my favorite parrot youtube channel

Hope it can be a help to you!