Hi N.,
Congratulations!! I am a huge advocate for breastfeeding. My daughter was 3 lbs 9 oz when she was born early and I pumped and pumped and did everything I could to ensure that I could breastfeed her and I did, exclusively for over 6 months, until she started having problems thriving because she wouldn't drink enough and I became so exhausted trying to make her healthy that I was in tears constantly and started feeling overwhelmed.
I think it's wonderful that you want to continue breastfeeding while pregnant. It's very selfless of you and if you can do it, great. But I think you also need to consider the needs of your own body and what it's going through right now nurturing not just one baby, but two. In my opinion, unless your Doctor feels very strongly about it, continue to nurse if you feel up to it. But don't feel bad, and don't let anyone else make you feel bad, if you are just too exhausted to continue. You have a lot with a 2 1/2 year old, an 8 month old and two more on the way! Being a great Mom is more than just what you feed them and if you become overwhelmed you might not be able to be the Mom you always have been.
I just wanted to say that, I feel like sometimes, as Moms, we are guilted way too easily. Breastfeeding comes very easy for some, but not for others. I went through a lot to ensure my daughter received breast milk, and I would do it again, but not to the extent where I feel I am lacking as a Mom in other areas. We're not superwomen, we just have super strength love for our kids. :)
Now, onto your question. :) I would talk to your pediatrician about weaning your baby, if that's what you choose to do. What I have heard is that babies don't usually care what's in the bottle, but that doesn't seem to be the case with your little one. I have heard about people mixing formula with breastmilk to get them used to it slowly (starting with a small amount of formula and working up to only formula), but I'm not sure if that will work. I think all babies are different, and really, you will just need to try different approaches until one works. When my daughter started rejecting the bottle (didn't matter what was in it) we got to the point where I had to leave the house for a few hours and my husband took over. The idea was, if she was hungry enough she would drink it, and it was heartbreaking to even prepare ourselves to do this, but it only took a few hours and she started drinking the bottle. Again, I would talk to your Ped. and see if you can get guidance there.
Sorry I couldn't be more help. Good luck and take care of yourself and all your little ones!
C.