Oof, three weeks in with flat nipples is really tough, mama, a few of us went through this too and it's such a mental load. Since your output is dropping and baby's struggling, I'd really suggest checking with a lactation consultant or your OB soon, kasi they can assess if your nipples need conditioning exercises or if a different shield size/type might work better for your specific shape. In the meantime, try hand-expressing before pumping to help with supply, at least that way you're not stressing as much about output going down. Some mum friends swear by the Mama's Choice Nipple Cream for conditioning, it's pregnancy and breastfeeding safe, and massage + cream helped them get baby to latch eventually, though I know shields are your main concern right now. Don't give up on latching if you still want to, but pumping exclusively is also valid if that's what works for you and baby, the most important thing is baby's getting fed and you're not burning out.