You can't have a positive relationship with someone who is constantly negative. Op, you had a positive relationship with your gps, that's great.
But irrelevant to your situation here.
You must challenge mil and her comments. Everyth me she makes them.
You are on borrowed time with dd. she will pick up on this negativity anytime now. You either deal with it, get the comments stopped, or remove the situations that allow them.
Your talk of the child's mental health and the negative impact on the child lasting a lifetime and reverberating through the generations is quite OTT in my view.
That's because you have absolutely no idea of what you are talking about/saying, and have no relevant experience NanaNina, you do this over and over. Your experience in some things may be useful in some instances, but when talking about MIL and negative behaviour you show a staggeringly blinkered and irresponsible attitude. You are fortunate to have lived a life without negativity/toxicity.
I don't have a mil, only a poisonous mother, and I've had the drip feed shite from her, and crappy little comments from my negative and critical father. The negativity and criticism they brought me up with knocked my confidence, and my sister's. At 3 id have been oblivious, but I remember situations at infant school that were related to their comments.
They carried on in the same vein when I had my ds. I have nothing to do with either one of them, and neither does my son. We're both better for it.
Yes I'd love to have the relationship with my parents that I had with my grandmother, but that is simply impossible. To allow them access to my son would hurt and damage him
Your situation op is not as extreme as mine, but it could end up being so. Clamp down on it now, and you have a chance of saving this.
I can't see you will get very far without Dh support though.
I know this is awful, I wish it wasn't, but you have a responsibility to your daughter and you are the only person that can prevent this.
She will gain in confidence, because she has you. Sometimes we have to be the mummabear, we have to make ourselves heard.
Good luck, you can do this