I was you! My baby suddenly got it worked out at 6 weeks and breastfeed for over a year. You may not have that outcome, but I know that awful treadmill of 6 weeks of trying to bf directly while pumping every three hours, washing pump supplies and bottles non-stop, and never sleeping.
It is a *very* different experience than that quiet stretch that some newborns and new moms have. It was really pretty awful.
Hang in there! And continue pumping and trying to breastfeed — or not. What your MIL is correct about is that things will work out okay in the end, partly because formula is okay, and mainly because your baby has a dedicated mom who is working hard on his behalf.
Your MIL is not right to make you feel like she is disrespectful or your choices and efforts. It was a boneheaded move — you know where to get or how to ask for formula if you want it. No doubt she her intentions are good, and she thinks your problem could be ‘easily’ solved, but, yeah, she should keep out of it.
With that said, let it go. It’s a small mistake from someone from a different generation with a different perspective. Just view it as her wanting you to have all options readily available.
Enjoy your new baby! (And get counsel from the Breastfeeding Center if you have not already.)