J.M.
As she gets older and eating doesn't wear her out so much, she'll become a more effective nurser. As long as you keep pumping so your supply doesn't go down you likely will be able to ween her off the formula (this is almost exactly what happened with my son. I was also able to drop the nipple shield after about 2 weeks). He went down to almost no formula at about 3-4 weeks, and had only the occasional bottle until I was ready for him to start taking more bottles at 3 months.
More importantly, if you can't wean her off the formula, that's okay too. It's not a failure on your part, and she will still be healthy and happy. You nurse her as much as you can and as much as she can do and that's all you can do. As a fabulous lactation consultant said to me when I was agonizing over supplementing "the most important thing is to feed the baby." You're feeding the baby. Keep up the good work.