Hi OP, congratulations on the birth of your little one
I echo what previous posters have said - very well done to you for expressing your breast milk. It is not easy, and you sound like you are doing a wonderful job. You should be very proud.
I feel so sorry for you that you are being forced to put up with this. It always astounds me when I hear about new mothers being treated like this by women who were once themselves a new mother, and therefore should be capable of
showing a great deal more empathy and understanding during this time.
Your post makes me feel so angry and upset on your behalf. How dare she treat you like this. She is wrong on so many levels. And how dare your husband let her walk all over you. You are priority. Being a grandmother does not trump being a mother. You are in charge here. I totally understand not being a confrontational person, it is difficult at the best of times, let alone when you are at your most vulnerable state having just given birth.
However, that being said, you must find the strength to assert yourself as the only people being hurt in this is you and your baby. Do not think twice about offending her as she clearly doesn't give a toss about your feelings.
"No I will not mutilate my child and that is the last I will hear about this, it has nothing to do with you"
"Please keep your advice to yourself, I would like to figure out how to do this my own way. If I need help I will ask"
"I am following the current guidelines by medical professionals"
"I am fed up with this, please give me some space"
You need to send her packing OP, go and stay with your mum who has your best interests at heart. It will be the best thing you could possibly do.
Sending you all of the positive energy to deal with this, you can do it, I hope things get easier for you and you can enjoy this precious time with your little darling. Take care xxx