Trouble latching now that milk is in/ nipple shields

auburnbride2be

So DS is only3 days old but has been latching well until last night. The only thing I can figure is my nipples aren't very long or large and now that my milk is coming in, my breasts are more full making the nipples seem smaller. We tried for literally an hour with breaks and I finally gave him a bit of formula last night. He was so frustrated and screaming bloody murder. Today we were back at it and I tried a nipple shield which totally worked. I asked about a nipple shield when I met with the LC an they said I didn't need one. Now that I am researching it, they say not to use one long term. Does anyone have any insight about the latching or nipple shields? I really want to EBF. My first LO would never latch so I exclusively pumped. I really want to avoid pumping exclusively.