We've been doing it for 1.5 months, the baby eats through a nipple (fl…

We've been doing it for 1.5 months, the baby eats through a nipple (flat, sucked-in nipples). She quickly gets tired of sucking and falls asleep, so she is often hungry and wants to sleep. That she isn't getting enough to eat, I want to switch to mixed feeding. On one hand, there are colic with or without, on the other hand, it's not harmful to switch to mixed feeding. Purely Gv doesn't get enough to eat. Purely IV conscience bothers me.