@anamelikenoother pleass be gentle with yourself, I am sorry you are feeling like this. I am breast feeding my baby, so I don't want to sound anti breast feeding, but the most important thing is that you feed your baby enough, so don't feel bad in any way about combi feeding or even exclusive bottle feeding (although it sounds like you would like to breast feed so I hope you can if you want to). I combi fed for the first 10 weeks with my baby due to supply issues. I tried to pump, bottle feed, formula feed. It was such hard work. Then a friend advised I just let our baby cluster feed whenever they wanted, have forumla at the end of the feeds if I needed to and only then pump (although to be quite honest I only managed to fit in pumping if my DH was able to feed our baby the forumla as I only have one pair of hands). This helped me, and by about 10 weeks he was EBF, but it might just have been luck and I didn't have an older baby to care for. Also if you have been advised by a medical or midwife team to give forumla in a specific way, you should obviously follow their plan for you. I am just saying what worked for me and we started supplementing our baby really early on, as I was struggling to BF so much, so he had no issues with weight.
We also had a late diagnosis of tongue tie. I was convinced there was a problem, but all of the midwives said it was fine, so I stopped asking for him to be checked for it. Finally we paid for a private assessment and once his tie was sorted he fed so much more easily and it didn't hurt me, so I was able to let him cluster feed.
Your situation will be different and you have already breast fed one baby, so you know more than me. What I would say is please don't beat yourself up about giving some formula. What is most important is that your baby is well fed and you are not too stressed by whatever feeding method you choose. Also, I would say please get some professional help with breast feedng if you want to from whoever you can access and feel comfortable with (midwives, health visitors, local infant feeding team, the NCT breast feeding line, peer support, lactation consultants). Our local council had a service we could access for free who were so much more helpful than the midwives (who were lovely but just told me the pain I was experiencing feeding was normal which was not the case). I would say if one service is not helpful try another. Unfortunately we do not always have consistently good breast feeding support in the UK..