OP, I feel for you. I've had to supplement with formula since day 5 of my son's life when he was readmitted to the hospital for jaundice and higher than normal weight loss. This was before we found out that I have breast hypoplasia (insufficient mammary glandular tissue in my breasts), and physical cannot make enough milk for my baby to thrive on alone. I was devastated...but with time, I've come to realize that a healthy baby boy who is gaining weight appropriately is the main point here.
Don't beat yourself up about this. In the meantime, just push the fluids as much as you can and keeping nursing/pumping along with the supplementing. Have you tried fenugreek to up your supply? Also I've heard blessed thistle, goats rue, and eating lots of oatmeal can help with this too. Hang in there, momma. You're doing the best you can, and that's what counts. What really matters is that your baby is happy and healthy...no matter if it's on BM or formula.