Girls, it's been two months, how can we get used to the nipple?

Girls, it's been two months, how can we get used to the nipple? Because I constantly suck my fingers until they turn red, even though I just sucked on the breast recently. To start, we bought a simple silicone nipple, but she doesn't take it, like she spits it out. Maybe buy a more expensive nipple? Or are there any ways?