You must not blame yourself. You have done SO MUCH to give her the best start and to look after her. You are doing SO MUCH more than what exclusively ff or bf mothers have to do. Expressing, bf'ing and topping up is a KILLER and no one knows that until they have done it themselves.
I have done it twice. The first time I got to day 3 before giving DD formula when she was jaundiced. She had screamed for at least 12 hours before she settled having had formula. I had tried so hard. The following weeks were spent topping up with formula and expressed milk. I gradually increased my supply and by 5 months I was able to exclusively breastfeed her. It was a real battle and her weight struggled throughout. I stopped at 6 months because despite my efforts I was too exhausted and overwhelmed to carry on. It had all taken its toll and was just too much emotionally.
The second time did not go so well. I bought a really good electric breast pump "just in case". Night 2 we gave formula because we recognised that she needed it. I spent weeks and weeks on minimal sleep from my round the clock pumping. I recall one week sleeping no more than about 5 hours. I was hallucinating and with 2 children to look after it was not sustainable. By 3 months I accepted defeat and moved to formula. The relief was instant.
I'm pregnant again and terrified. I desperately want to breastfeed but I struggle with it so much. I don't understand why. I have always wondered about my supply (getting 4oz in 24hrs) but health visitors have always told me that babies are more efficient at feeding than pumps and that what I express is no reflection of what the baby gets. I get the fear that what if I ebf and then the baby doesn't gain weight, what if I cause harm. What if my persistence and determination is only detrimental because no matter what I do, my supply or something is just not good enough and I'm doomed to fail. Why do I care so much? Well because I want what every mother does, what's best for their baby and the NHS says ebf is best. When you see other women doing so well at it and finding it so easy you just feel hopeless and while you have lovely people on forums who assure you that it can be hard and having a happy mother is more important...it's always that one post where someone has said "breastfeeding is great and so good for baby, I don't understand why all mothers wouldn't want to give their baby the best" that sticks in your head. There is far too much pressure to succeed at breastfeeding.
You tried SO hard and you are still trying hard. You are obviously full of regret. So am I. For me the way to move on was to switch to ff an start enjoying my babies. I knew that one day they would be older and it would not matter. My first who definitely had more breastmilk than my second, gets I'll equally as much as her sister. I'm not saying there aren't benefits to bf, because there are. I'm saying that I personally don't feel the benefits don't outweigh the negatives in the situation where the mother is running herself into the ground.
I really hope you can make some peace with this. You did everything you were told to and more.