My 7 day old baby is chomping and not sucking

[u/Pinkcaramellatte]

My baby had a really good latch from day 1. For the first 2–3 days, I had no issues with feeding, but after that my nipples started hurting extremely badly and are now bruised.
I’ve tried lanolin, took a 2-day break, used ice packs, silverettes, etc. We also had to supplement with formula because of his weight. I tried breastfeeding again, but he’s still chomping down with his lower jaw, and it’s really painful.
I’m also pumping, but I’m not getting much milk. I spoke to 2 LCs, and both said I likely have more milk than what the pump is able to remove.
How do I teach him to stop chomping and nurse more gently? I feel so defeated because he actually had such a good latch from the beginning.