4 kids under 5, you are a rock star!
I know why you feel the need to give them what you gave the others but you have to give yourself a break, the circumstances aren't the same. With the best will in the world none of us parent our 3rd/4th kids in the same way we did our 1st, sometimes we do better, sometimes we do worse, but we are doing our best!
I don't have much advice, I think you are a hero for trying. Getting breastfeeding established with one is a mission, I can't imagine two. I just couldn't read and run. If breastfeeding is important to you then please reach out to an NHS peer support for advice or, if open, get to a breastfeeding group. Feeding two must be a different game and I'm sure those with experience have tips. But please please don't feel guilty. I am a madsive advocate of breastfeeding (mostly for the benefits for mums) but I am also bloody sure that noone can tell who had formula and who had boob when they are all on the playground.
From a practical perspective have you tried the multi mam compresses for the pain? They are aaaaamazing. You get them from Boots. They aren't cheap but I would cut one in half so they'd go twice as far. I also kept them in the fridge.
Also, for me, the horrific stinging burning pain lasted 10-14 days per baby. Its sooo so grim, but remembering that it will end soon helps (whereas with my first I couldn't believe that they would ever heal).
Good luck, but please lose the guilt!