Get the locks changed (immediately) and don’t give her a key.
Ignore crying or emotional blackmail.
If she says I won’t see you until x date say “that’s lovely we’ll see you then”
All this will be excellent practice for when your baby is a tantruming toddler.
Set firm boundaries and stick to them.
Never, ever, ever give into a tantrum regardless of the age of the person doing it. You are just teaching them that the manipulation worked so they’ll keep doing it.
You also need to have a conversation with your DP.
Of course he loves his Mum and of course his relationship with her is important, however his primary family is now you and the baby.
If he has to choose, he needs to choose you.
Once you’ve got that settled do your best to make sure he never (or very rarely) has to choose between you.
Be cheerful and polite but be happy to say “no” or disagree if necessary without guilt.
If she starts yelling at you feel free to politely and calmly leave her house.
She isn’t in charge. You are in charge of yourself and your child.
You are the gateway to her grandchild so you hold all the power here. You don’t necessarily need to ever exercise that power but I find it’s useful to remember it.