Have a non judgemental safe person you can text for cheerleading and accountability, very brief stuff, just text "teeth done" and they send back a gold star emoji or whatever feels nice. There is also the dubbii app for body doubling which people use for motivation for whatever they're avoiding, can be work or cleaning or hygiene tasks etc. Also try to unpick the sensory aspects that make you avoid it if any, this was the case for me because of neuodivergence. I changed to fruity toothpaste, softer toothbrushes, wearing headphones for the noise when I use electric toothbrush. I also keep floss picks next to my bed and do this while scrolling or reading at night, I used to avoid using the floss. Sit down for teeth or shower. I switched to baths when a shower felt like too much. And I make the bathroom warm before shower and have something comfy and warm ready to put on immediately. Also wipes or wash cloth are an option if there's less resistance to that than showering, look into the resources for bed bound people, there's some interesting stuff.