I don't necessarily think of it as an 'inner critic', but I guess that's what I'm being taken over by when I get all filled with wordless shame in session and just curl up in a useless ball and can't express myself.

Covering myself with a sheet somehow helped a lot with that last session. I realise that is an unorthodox approach and perhaps something that only works for me (I haven't worked out exactly why yet)... previously to that, I generally had to be distracted by something nice or funny to come out of the shame-hole. I suppose that usually allows a different part to take over.

Sorry, I'm not sure if any of that is remotely helpful!

I would like to suggest that perhaps telling it to 'piss off' isn't actually the best approach. I know your inner critic seems like a total arsehole but it has its reasons for behaving the way it does. It probably thinks it is protecting you somehow. Could you try to dialogue with it, ask it what it wants, why it won't leave you alone? Something like that? Or get creative - draw a picture of it, sculpt it, see what comes out of that...