I’m not sure if refusing to talk to your dad is a good idea. However I’m also not sure if having this talk with your dad is a good idea either!
I am sure that you shouldn’t talk to your step mother about it again though! Or be pushed into having both your father and step mother together ‘talking to you’ (or to put it another way, ganging up on you).
What is your dad normally like? You say you are close and it would be a shame to let this situation get in the way. However, what happens normally if your step mother decides you are in the wrong for some reason? Has she ever behaved like this to you before? It’s interfering, and treating you like you aren’t capable of making decisions like an adult. Does your dad ever stick up for you or go against his wife?
If this is part of a pattern of behaviour where your step mother is successful at putting you down, or making your father doubt you, then I’d be saying to stay well away from the pair of them.
What are dynamics like between your father, and step mother, and your mother? Could this be about taking a pop at your mother through you? Or jealousy that your mother is involved and really helping you out?
You don’t want to open yourself up to any unpleasant scores they want to settle or battles they have been engaged in for years. You are not cannon fodder! You are too vulnerable right now to cope with dysfunctional family dynamics that may have been going for years and highly unlikely to change now!
However if your father has shown before that he is capable of making decisions and having opinions that go against her, and he also shows he can put his bond with you, his daughter above and beyond his wife... then maybe it would be ok to talk to him (on his own)?
I’d definitely prepare a list of points that allow you to make the situation clear, and that stop you from getting too upset to be able to put your points across clearly.
Then say calmly and clearly that there is no reason to criticise your parenting. And that it’s a very hurtful thing to do when there’s no foundation to it. It’s not a supportive thing to do. And it’s not going to change anything, as there is nothing bad to change.
I’d personally ask him why one day a week to close family (the same person as well!), has become tossing the baby around and the baby spending more time away from you than with you?! It just doesn’t stack up. It’s a nasty accusation and it sounds like things have got blown out of proportion in your step mothers head ... so even though it may have come from a good place originally (trying to be nice here!), it’s ended up being really unhelpful and upsetting.
Also how is your dad about understanding and caring about your mental health struggles?
If he’s caring and supportive, perhaps remind him that your mum is helping out so you are able to get better and continue to be a good mum. It’s great you have support and you are being a really good mum by using that support... after all ‘it takes a village to raise a child’, not one vulnerable mum on her own and draining her health!
Say you don’t want to speak about it ever again, and that you need support, not unfounded accusations and screaming arguments based on nothing! ‘Because I’m a good mum I know I need to keep my mental health up and protect myself from things that may harm it, so I’m sure you’ll understand that I cannot have a repeat of this ever again. And of step mother can’t get over whatever caused her nastiness before, then you’ll have to be careful about exposing yourself to it/ her again’
And just in case:
‘This is stepmothers problem, not mine. Her opinions are not based on any truth, and are not helpful in any way. So I don’t want to hear anything more about it. She needs to deal with her attitudes away from us, and we won’t let her take it out on us again, because quite frankly, it’s nothing to do with us at all’.
Good luck. Is there anyone else you can talk to about this in real life? Would your health visitor or of you have one, your counsellor be able to help?