Re: young wife older husband...
I'd suggest this is the tip of the iceberg -- there is a lot more underneath what is visible here, and I'd be doing everything in my power ot get underneath and see what's hidden, expose it and start dealing with it.
No she can't have her cake and eat it too, but the reality is you need to do some work together to address her blatant need to get in on some of what she is missing out on as a 21 y.o. wife and mother who resents her husband because he isn't also 21.... She can't and won't ignore her feelings here, and only unpleasant things will come about from this. Her suppressing it because 'she's your wife and a mother now' will only lead to bigger resentment and larger problems down the road. Perhaps there are much smaller compromises you can make now, together, to prevent way more painful issues down the road?
Get yourselves to dealing with this, head-on. I'd strongly urge MC.
Obviously health of the baby must come first, and if she can't do that on her own while breastfeeding, you have every right to insist and control it. Beyond that, you might find ways to give her a little more as she struggles through a tough time that yoyu both bought into when you got married and had a baby.