Hey lulu, I don't experience flashbacks using the shower, but I do experience them from using the toilet.
During my abusive relationship I would hide in the little room that had the toilet in it; just sitting on top of the toilet seat and not using it.
So now, every time I sit down to actually use the toilet, I'm right back there waiting for the door to be burst open any moment by him. And physiologically, by body just won't go anywhere near voiding until I can get myself to ground.
Grounding there usually involves giving myself a pep talk quietly out loud. Telling myself that I'm safe, and that it's 2020. In those pep talks I'll also call myself by my forum name, which may sound a little silly, but it's because I signed up here after leaving my abusive relationship, so I was never "bellbird" in that time.
Sorry if that's tmi, just wanted to say that I really get the frustration and the feeling of ughhh this is so ridiculous on a daily basis, that you're going through.
Grounding in the shower would be my big suggestion. There are several sensory avenues you have access to in a shower. If you're ok with scents, maybe a bottle of body wash with a calming scent that you can smell when you find yourself starting to dissociate.
Or a little squishy or textured waterproof toy that you can keep in the shower and grab if you need to to ground yourself in the here and now.
Maybe you could listen to music while you're in the shower -- songs that make you happy, songs that soothe you, or my favourite trick which is songs that have been released since whatever you're flashing back to happened. That way you definitely can't be back there.
To tackle this issue at the root cause, one of the best methods would be EMDR with a trained therapist, though I get that therapy is a little different and less accessible in these current times.
Regardless, you're not alone. And I'm sorry for what you're going through :hug: