Worried about autistic overwhelm/shutdown during EMDR

I was diagnosed with Autism later in life, and am now engaging with EMDR, about to start formulation. I am really worried about overwhelm/shutdown happening within a session. I tend to go from 0 - 100 very quickly and, thus far, don't really recognise any distinctive triggers that give me a hint or chance to stop it. I find it really embarrassing as I can get hysterical or completely catatonic and I'm almost feeling anxious about that alongside everything else.

Can anyone shed some light, tips or how they navigate this?