porridge helps your supply as someone has told, so will anything with otas init, e.g. alpen, flapjacks etc. I took a box of flapjacks into hosp when I had DS" and I had the same problems with feeding as with DS1.
The flapjacks helped me produce tonnes of milk and I expressed so that I could see how much he was getting. (isssues with weight loss etc).
Turmeric helps you produce milk too, I spoke to a NCT counsellor with DS! as he wasn't gaining weight (now a very very tall, extremely clever 5yr old) and she advised me to take turmeric and eat lots with oats in it. You can get turmeric capsules in Holland and Barratt, order online if needs be.
I also sometime s gave Ds2 formula during the night (kept a bottle of boiled water in an insulated holder beside the bed, along with a pre-measured container of powder. The water was just at the right temp by the time DS2 woke up hungry.) I'll be very honest and say this was to save me getting up and going downstairs to warm up EBM (you can't mocrowave it) and i didn't feed him myself during the night as he didn't feed well, although was there for ages. (feeding issues I mentioned earlier).
It also meant that DH could feed him too sometimes, and he didn't wake everyone up crying as his bottle was ready in a minute. I was more relaxed about BF and FF with DS2, was adamant with DS1 that he would only have me. But then, I had loads of time to sit and feed him (had weight gain issues with him too.)
I bought a double electric pump and expressed several times a day. It was hard hard work as I had to satnd in the kitchen and lean over the worktop, couldn't do it sitting down. However, I produced loads of milk, which I could actually see. Ds2 probably got the same from me in 3 months as DS1 got in 6 months!!
It's a hard time, try the turmeric and eat loads of flapjacks. Consider expressing and let Dd carefully feed baby?? good luck anyway.