Clare oh I really am just so sorry you’re going through something so similar. For me it was so long ago that I can talk about it like it’s something from a horrible book, I’ve emotionally cut it away, I think.
The advice I would give to you, aside from the obvious ‘stand your ground’ type stuff, is to arm yourself with knowledge and understanding of your baby, in order to gain confidence.
Listen to audiobooks like Philippa Perry’s ‘The Book You Wish Your Parents Had Read’. It’s about not following fads or allowing toxic parental relationships or mistakes to filter into your relationships and bond with your own child. It’s about the more primal needs of your baby and explains it all really well. When you research something and find evidence that lines up with your own feelings and beliefs, you gain the power and confidence to uphold those beliefs. When it stops being a feeling, and you understand it as a fact, it’s easier to push against the people around you telling you that you’re wrong. Arm yourself with information about why babies need to be as close to their primary caregiver as possible in their first years. Why separation of mother and baby can cause huge psychological and emotional distress to both.
Whilst I think that you should of course be able to say “I’m not comfortable with that” and be heard, clearly you’re being worn down. When you’re totally worn down, the frantic anxiety starts to kick in and you second guess yourself. That’s when the strength that comes from knowledge can be helpful. You’re not being silly or precious, we are biologically designed to want to be close to our babies. Our babies are completely helpless without their mothers on an evolutionary level. I understand that we have bottles, lovely convenient things and all sorts now so babies can live without their birth mothers, but it’s not the gold standard for Mother and child. We are programmed, hard-wired, physically built to be with them. It’s the simplest thing in the world, but people and their self-centred narcissistic ways will always try and derail even the things that everyone else instinctively understands to be sacred (like that a baby should be with its mother!), yet they butt in anyway with their own selfish demands.
Nobody has the right to pressure you in this way, but they are doing it, and you’re a new mum with a small baby, so it can be extremely difficult to be assertive and tell everyone to just fuck off! I don’t expect you feel up to having a showdown with MIL, so just take the scenic route, learn some interesting stuff along they way - and hit her with some facts next time she starts pestering you. Be warned. If she left your DH with grandparents from a tiny age, she will have even more motivation to pressure you into it, to validate her own decisions as a mother. That could make her cling onto the idea a bit tighter.