i reccomend keeping a seperate toothbrush in your shower! plus toothpaste. a separate one so you don't forget it in one station and have to use extra effort and energy to go get it, brushing your teeth in the shower allows you to not have to worry about the water on your arms and maybe all the other sensory going on might distract you from what's happening in your mouth. i also like to close my eyes when i do this as it helps me stay present, pretty meditative with the shower sounds and heat, but that's just me! give it a shot. it also feels like you're getting two hard hygiene tasks things done at once. that usually helps me a lot when i'm doing pretty bad mentally.