This is the thing about people (especially elderly or mil specifically) who dont understand the hardship of breastfeeding. They think milk just comes freeflow and that everyone should have milk that they forgot most people grew up formula fed and is equally healthy and strong.
There is no scientific proven that kids that don’t eat enough home cooked food will be any weaker than those who eats home cooked food everyday what. Or maybe you can ask her if she wants to take up the cooking role when baby is able to take solids? It’s not like everyone have enough time or energy to use every single day to cook all 3 meals of HCF, especially those who needs to work and have no other help.
And yes, being a happy mom and getting our sanity back is more important than breastfeeding. Stress is the biggest milk killer. So while they care about baby’s wellbeing, they should think about the milk maker (you)‘s wellbeing first.
You are already doing your best so try to not let her get into you.