How can I become more assertive, especially with my overbearing MIL?

I don't know were this issue stems from in my life but I've always had trouble being assertive. I hate confrontation, saying no, expressing my feelings etc.

Im a massive people pleaser and push over when it comes to myself. I really struggle to stick up for myself because when I do im always considered the bad guy. sometimes I bottle it up for so long when it does come out I explode and make that person cry or something.

Its followed me my entire life I try to avoid upsetting people and in the it makes tge situation worse because someone else ends up intervening such as my husband or best friend.

I have a 3 week old baby girl and im finding im gonna have to be more assertive for her. Id do anything to do right by her and protect her. Im having trouble with my mil atm as she's being extremely over baring and intense. My husband confronted her about it and she didn't really respect the boundaries and kept making bitchy comments so he took her to one side and spoke to her again about it which he got upset about and just said stupid things like don't expect me to babysit again then when I fact it was more a case of kidnapping sometimes.

The women has a really fucked up idea of boundaries she thinks giving me help with the baby and being a normal grandmother is overstepping but seeing her everyday and helping herself into my house whilst im sleeping naked or breastfeeding isn't.

she's just very odd woman if im honest but she's done a lot for us and has actually been through a lot so I don't like talking bad about her.

I feel i can't tell her the truth and if i do it either gets dismissed or im made out to be in the wrong.

My husband spoke to her again today and we've all decided but we need to all start better communicating with each other if I dont want something I've gotta say it but ill still get dismissed or questioned i know I will.

I've been coaching myself on even how I can respond to her when she's being like this
Updates
3 h
My husband had a world with his mother today. We hugged it out and she apologised. I feel so much better
How can I become more assertive, especially with my overbearing MIL?
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