I know Breast is best and I managed it with dd (2nd child). But I went through hell with ds - eventually it was established that he had an oddly shaped palate.
You have done incredibly well to keep this going for 9 weeks. There are more benefits but your baby has had the majority of them. There is no crime in giving a baby some formula although no-one really tells you that.
DS was hell to feed and we started giving him some formula when he was 8 weeks old. At that point we realised he must have been sooo hungry - he slept and settled all of a sudden and mum was happier and baby was happier and there started a virtuous circle.
I felt guilty for years but it didn't stop him from growing to 6'2" or making the first XV. He has no recollection of receiving sub optimal care as a tiny baby - I do but I got over it and with the birth of dd realised they are all different.
He remembers: first graze and a hug, Santa's footprints on the carpet, his first goal, his first try, his first part in the nativity, that Elizabeth pinching him on the carpet in reception, the first time he got drunk and his mum cleared up the vom, his first time on an aeroplane, going to Disney, and funnily enough now he's 19 and has had a few mates who haven't been loved totally and unconditionally he has actually said that he realises how much me and dad have really cared for him and his sister.
It's early days OP. Parenting is about more than the first few weeks and a lot more than about breast feeding.
Do what's right for you and baby Puddleduck. Good luck and don't beat yourself up too much. This world is just getting enjoyable as your baby gets to the point where she can start giving something back.
)))hugs(((