Oh OP, I can massively relate to your post. I had to triple feed DD1 for months as she was drifting below the 0.4th centile. She had been in NICU - i already had huge guilt feelings from that and not being able to fully bf massively destroyed my MH. It's good to hear you are under the relevant services. I'd had fertility treatment to conceive and i remember saying to the NICU psychologist that i felt like a failure of a woman - i couldn't get pregnant in the first place, I couldn't keep her safe in labour and then I couldn't successfully feed her. It was a horrible time.
I can say that sometimes it takes a while for the tongue tie division to start making a difference, so that might improve in a bit. But what really helped me was to disengage a bit from social media and to some of the baby groups for a while as well. The whole infant feeding topic is so polarised, i decided in the end to figure out who I wanted to trust (whether that's the feeding team, or an LC) and just listen to them without all the external noise. Once I filtered a lot of other stuff out, it became easier to learn what my own instincts were.
Without top ups, I now believe my DD's weight would have continued to drift, her sleep affected, her development affected etc. She was a very sucky baby and would have stayed at the breast for hours, not drinking properly, getting tired and that would not have been good for her. It would not have been for the best to me to try and get to EBF. Not to mention the fact that I think all the extra expressing was a massive factor in my declining MH. But I would never have recognised that at the time when I was in the thick of it and it is only now it is in the past that I have been able to see the wood for the trees.
I also totally understand the feelings of rejection. I have no real advice other than to say from someone who has come out the otherside that it does get better. At the stage you are at, every day feels so hard but day by day, your feelings will change. I used to set myself little goals - BF for one more week, then the end of the month, then 3 months, then 6months etx.
In hindsight, I should have also considered seeking out some PND help to help process all the feelings I was having - maybe some antidepressants or CBT or something.
I've since had DD2, who also has some feeding issues, a 90% tongue tie that wasnt found until she was 8weeks and other issues that means a feed takes 90mins. This time, I accepted the need for top ups with a more positive mindset, which I feel means i haven't had the same MH issues. Kind of because I approached it differently? I also refused to do the 8 x expressing this time, which has hugely helped with MH and sleep!
Both babies have still been having some breastmilk at 6months, which given where we were at 4weeks with them both, I am now very proud of that achievement. And I hope in time you will feel the same. My amazing HV reminds me at our fortnightly weigh ins that every drop of breastmilk helps so not to flog myself if the balance between breastmilk and top ups sways more to the formula. I've decided to stop breastfeeding at the end of the month (DD2 will be 6months then) and this time i feel really at peace with my decision (unlike last time).
Sorry this was an absolute mammoth essay but your post just resonated so much. You are not alone in how you are feeling. You are doing brilliantly and you will get there, even if it ends up looking slightly differently to how you expected when you were pregnant 💪