Sweetie I was so sorry to read that post, the only advice I can give is that your responsibility is to that little baby. Your Mother in Law is going to have an opinion on everything about your child for the next 30 years and you will have to tell her to back off at some stage. It is a real pity that she is starting already. Your partner needs to make decisions with you about your child not with his Mother. He is a Father now before he is a son.
Your Mother in Law may have some out of date information about formula/breastfeeding too. In the 50's and 60's there was a very effective advertising campaign by formula companies to make breastfeeding seem like the inferrior option, something you did if you couldn't afford formula. We have since learned that breastmilk is a very healthy choice for your baby. (I breastfed but I think whatever you choose is great except coffee or brandy maybe.) If you have the strength to take on your Mother in Law and Husband right now, show them all the information that shows you are making a WONDERFUL choice for your baby.
Look after yourself and your Angel and don't let anyone bully you into doing something you are uncomfortable with.