When LO was about 2 weeks old my OH went to work and I was left on my own for the first time since she was born. He left me sitting in bed feeding her, he came home in the afternoon and I was still sitting in bed feeding her. I had only gotten up a couple of times to change her nappy, the rest of the time was spent feeding, burping, getting her to sleep for 5 mins, then starting again. I hadn't eaten or drunk anything and was desperate to pee, shower and get dressed! After that I learned a lot of tricks like leaving snacks and drinks where I could reach them. At 6 weeks she was feeding 8 to 10hrs out of 24. It got much, much easier for me at about 8 weeks. My supply got better and she got good at sucking.
I think you are doing amazingly, especially with another kid to look after. If you can and want to stick it out it will get easier, you are so close to feeling the benifits of all the hard work youre putting in. In the long run BFing is so much easier than FF and is very rewarding but the first weeks can be rough.
How much formula are you feeding a day? If it isn't a lot you may just be able to cut it out right away. She will be hungry and fussy for the 1st 24hrs but hungry babies feed a lot and that will increase your supply quickly. If she is getting a bigger amount you might need to reduce the amount over a period of time, either by giving less each time, or less often. I think a lactation consultant could give you some good advice on how to go about it. If that is too expensive for you have you could try a helpline from a breastfeeding support organization like the LLL.