3 weeks old still not back to birth weight

I really enjoy breastfeeding and I want so badly to solely breastfeed. But we have struggled from the start with latching and have seen our lactation consultant (who is amazing) twice a week. We started using nipple shield after a few nights of tears and struggle. Nipple shield

Worked great! But when we went for 3 week weight check and he still wasn’t back

To birthweight I felt so defeated. I have been feeding him around the clock and pumping after and bottle feeding what I pump. They think he has some sucking restrictions. Sent me home with formula to supplement:(( I am heartbroken I have nothing against formula but I want to do this so bad. Why is something so natural sooo hard?