The hardest thing will be to stick with it, because they ARE going to push back at first. They will want to put you back in your place because anything else would be uncomfortable for them. You have to decide to play a new role and completely abandon the old one. So when your mum says, "Oh diddums", you should just continue with what you are doing, i.e. say something like, "That's very insensitive of you, mum, and it upsets me when you do that." And when they inevitably make fun of that, just just repeat the same thing. Eventually they will see that they are getting nowhere with their insults and insensitive behaviour, and they'll stop. But it is going to require persistence, because in your family script that is the only way you know how to be around each other, and unfortunately you are going to have to do all the work here because they aren't going to help you do it. They are going to push back, because that's all they know how to do.

Don't give up. It won't be easy, but if you persist your parents will eventually realise that they are banging their heads against a wall, and they'll stop. And if they don't, just remove yourself from the situation.

With my parents, I have lived with the following two options: either sit and listen to the same criticism I have had to listen to for years now, or start an argument I'll never win. I'm sick of that script, and no longer want to play a part in it. But in order to rewrite it, I'll have to repeat my new lines until they stop trying to push me into saying the old ones. It will be hard as hell, and I anticipate a few stumbles along the way, but I will not give up until they accept that I'm just not going to play that role anymore.

Just persist, persist, persist!