I know you've had problems getting baby to take your expressed milk, and that can make pumping discouraging. One thing you might want to try is putting a bit of expressed milk in every formula bottle, so that formula masks the taste of breastmilk. That way your hard work doesn't go to waste, and even a tiny bit of breastmilk in the formula improves the overall nutritional value of the bottle. Ordinarily moms are told not to mix breastmik and formula because the storage guidelines are different- unused breastmilk can go back in the fridge, but unused formula must go down the sink. So if baby doesn't finish a mixed bottle, that means dumping breastmilk along with the formula! But in your case, your baby won't take breastmilk via bottle, so I think it makes sense to risk sending a bit of breastmilk down the drain than having your entire pump output get wasted.