ManyMonkeys - I'm breastfeeding my 4th DC at the moment (he's 6 months, other DC are 3, 5 & 7) and yes looking after 4 DC is exhausting, manic, busy & stressful... Which is why breastfeeding is brilliant as it makes me take time to sit on the sofa and relax with my baby ever few hours!
Women living through terrible conditions and traumas all over the world continue breastfeeding their babies. Needing to rest to maintain milk supply is not based on the evidence, neither particularly is eating and drinking lots; your friend is obviously comparing herself to a docile mammal like a dairy cow. I prefer to compare myself to a more intelligent mammal, such as a tiger, which of course also feeds its young. I'm a powerful protector of my young, nurturing them and providing for them, and an important part of this, for me, is breastfeeding them.
On a practical level -
Get a decent sling which you can bf in (I can't manage to bf them in a sling when they're tiny, but from about 8 weeks onwards I can). I used a wrap sling called a moby, it was about £35. Also, putting him in the sling to get/keep him to sleep at times when I'm busy with the others has been essential. He lives in it from 3.30-6 every day!
Bfing on demand isn't the same thing as not having a routine. I, for example, always make sure I feed and play a lot with DC4 in the early afternoon, so that after school he will sleep.
Once baby is born, you may well be able to get a bit of help each week/fortnight from a homestart volunteer.
Turn your other DC into little helpers; get them to fetch adn carry for you while your bfing, so that once the bfing is finished you can have a bit of time to focus on them.
Consider co-sleeping. I'm not sure how I'd manage to get everyone up and out the house by 8.15 in the morning if I was as regimented about putting DC4 back in the crib as I was with DC1!
Try to spend time with people who increase your confidence. If this is a once off then ask your friend to try to support you by telling you how well you are doing and will do, rather than the opposite.
Best of luck with the delivery, and sorry to hear about the stress with your ex.