first, this has happened to me with both my babies. With ds1, he was taken back into hospital at 7 days, he had jaundice and was getting quite ill. I didn't really get the support I needed to maintain breastfeeding while/after he was fed up to get him well, and although I tried to express for a while it never really worked, he was essentially formula fed.
In ds2's case, he was referred to the hospital at 4 days and readmitted at 5 days with a 17.5% weight loss, apparently a record (hospital claims they'd never seen a baby with such a high proportion loss). Gulp! I was devastated that it was all happening again as I was so anxious to make bf work this time.
This time I got the help of a breastfeeding counsellor working at the hospital, this is a new development since ds1 was born less than 2 years earlier. She's based on postnatal but came up to the Paediatrics ward every day to see me and help me to latch on etc. I managed to stabilise his weight on breastmilk but we were pushed into top-ups after 5 days as he hadn't put any on. However, doing it this way is horribly hard work and I imagine a lot of women just give up, as I still fed him for at least 20 minuutes before offering top ups, and often at rather less than 3 hours between the start of one feed and the next. We used the hospital's convenient readymade bottles with teats etc so at least we didn't have to worry about mixing and washing up and sterilising, but with that this approach is incredibly time consuming.
Once we got out I gradually phased out the top ups from 6 to 3 to 1 and then none. I think the last top up was 2 weeks ago. When he was on 3 the hospital told me he wasn't gaining weight fast enough and wanted me to use more, but I looked up figures for reasonable breastfeeding weight gain and I'd been given a figure at least twice that, ds was only a little under the more realistic figures. His gain has then varied a bit, and I panicked, but really picked up this week.
Last Friday I was convinced the hospital were going to demand the resumption of formula top ups, but then he was weighed by the GP at his registration appointment there on Wednesday, and by the hospital on Thursday. Both GP and consultant at hospital were very happy with his gain and general healthy appearance. No more suggestion of top-ups.
The upshot of all this? It's very very hard work to do, and you will have to be prepared to seek out and keep asking for support from the health professionals, if that's what you want, that is help to maintain bf or mixed feeding at least. But it is possible.
Do come back and get advice from real bf experts on here, and I'd suggest finding out whether there are any bf support groups in your area, seeking face to face help from breastfeeding counsellors etc.
What not to do - don't feel guilty. It sounds like apart from the mw who came by today you haven't been given adequate support.
Puppster and mrsbaldwin's suggestions sound better than the hospital's advice if you want to maintain breastfeeding rather than ff.
Good luck whatever happens.