I would agree with Livercake - if it's at the point of endangering her health, I would muzzle her.
This would give you the opportunity to develop a rock solid "leave it" command (by starting with low value items at home and gradually working up) as well as breaking the habit she has got into....
I would also experiment with a variety of toys... Honey behaves very differently with her frisbee than she does with a tennis ball, likewise with a squeaky toy...
What about a ball on a rope.... that is less likely to become boring as you can have a game of tug when she brings it back, rather than just playing fetch...
If she likes a game of tug, this could become the reward for leaving a stick...