frustrated

Annaliesa
I'm getting frustrated bc my husband and MIL keep saying that I should stop breastfeeding my son and switch to formula only so that we know how much he's getting at each feeding. I still want to do both bc it's best for him and financially too. My husband said something yesterday at the cottage and my MIL said I should stop so we can keep account but then I told my husband this morning I need to feed him bc I'm full and didn't feed for over 12 hours and I was sore. He said "oh you can still breastfeed if you want.." but makes me fee guilty bc I can't say how much he gets when I feed him. I don't want to be rude but I'm getting irritated bc it's always being brought up. I originally planned to feed him partially for 6 months - 1 year or so. 
What can I say that politely says that I'm aware of what they're saying however I am still going to feed partially as I am now so that this discussion isn't brought up every time? I've already been pretty straight forward and I love my MIL but I'm getting to a point where I want to freak out.. HELP