aitch.....stop feeling so bad about this......your are doing fabulously, you truly are.
I had a dreadful feeder in DD3. she was born at term, but was born with chicken pox and it had a 8terrible* effect on feeding and weight gain. In fact, she was 8lb at birth and then dropped right off the charts......on her 1st birthday she was 15lb 11, so she didn;t even double her weight!
her issues tho were not just feeding/illness (she was later found to have tongue tie, but too late to do anything with), but also severe reflux, sleepyness and later, what turned out to be dairy allergy.
I struggled for weeks with breastfeeding.....in fact we even had to go into hospital for help when she was 9wks and then again at 16wks (and you know......we did esactly the same with DS2......apart from the chicken pox, DD3 and DS2 were identical with feeding/lack of weight gain).
anyhoooo........I digress.....you want help, not to hear/read about me.
do you pump regularly (ie, once she has had her feed, you go off and pump a little more to encourage more supply). I assume you have had wonderful advice from tictok and co.......??
I think part of what you are going thro, reading your post stating that she feeds beautifully when you are at the BF place, is that at home, you have all the other distractions and your older DD, and so are not quite relaxed, so of course DD2 will oick up on that slightly, and also, you are just tense and fretting that she is not taking enough.......you know, that is normal.....I was the exact same, the only difference being that I had either 2 older children in DD3's case, and 4 older ones by the time DS2 came along......SO not conductive to relaxing peaceful feeding of the type you imagine yourself having with your sweet new baby.
your DD is gaining, so you are clearly doing lots right. yes, she is gaining slowly, but hey, maybe she would anyway if she was on formula......without cloning her and trying feeding differently you can never know.
you might just have a naturally petite little lady who just needs more time to perfect the feeding. it is just up to you now as to how long you want to keep going for....and it is ok whichever option you take. you have given 16wks.....`16 wonderful weeks more than many out there. you have put heart and soul into this, no one could have asked less of you, so feel amazed and proud of yourself.
do not forget either......you have also had to recover from a sudden and scary delivery, plus (IIRC) all that hassle from builders dong the upstairs flat (that is you isn;t it, suffering that hell??).......don;t knock yourself, you really have given 100% of yourself to her.
right now tho, you need to relax, stop feeling guilty that your are failing her......you are not, you truly are not!
if she has the odd bottle, so what. why not give her one a day.......one that your DH can give while you rest maybe, or have a bath, pump, have a little you and me time with DD1.
and if you decide to swap completely.....that too is fine, but do it for the right reasons (that you want to), not that youo feel forced to thru guilt, as you will always and forever wonder 'but what if I had just tried for one more day'.