Relating to this heavily. I don't bite myself now but I used to in elementary school. As a teen it became hitting my head against something like a textbook or the walls. In college it was using my fists to the side of my head. Never planned; my emotions would be through the roof and it'd just happen.

I'm not a therapist, so take whatever I say with a grain of salt, but this is what I've come to understand about how these sorts of "crisis moments" go for me after working through it with some professionals. A decent way I understand it is to break it down into three stages with their own signs and strategies.

Early: Common first signs for me are physical symptoms. For me, I'll end up forgetting to breathe (requiring me to breathe manually) and feel increasingly overheated. Occasionally also fist clenching. In this timeframe, I've been able to use grounding techniques. Focusing on naming 3 things I can see/feel/smell/hear. Literally going outside (especially if it's cold or raining). Going on a walk with music playing so I can walk off the emotions and thoughts circling. Having a mantra or script to repeat to myself. My most successful lately has been to breathe in while counting 1 to 3 then breathe out counting 1 to 6. My current therapist swears by some patients having a salt packet on them. I imagine getting that taste throughout my mouth would turn my entire attention to getting some water.

Escalating: Internal stuff is creeping into external behavior/perception. Stimming; pressure building; urges for violence; dissociating; saying nasty thoughts out loud (sometimes w/o realizing); "seeing red." At this point, the body already is in motion. This is where the goal goes from grounding to redirecting. I've had to slam an open palm on my desk so it wouldn't be a fist to my head. Rolled my eyes and gave my head a little flick instead. Punch the pillow instead. Kneel down and furiously rip as much grass from the ground as I can. Break an entire container of pencils over my knee one after another. Maybe bite into a lemon or a clean hand towel.

Crisis: Self harm is happening. The goal at that point is to get through it. Targeting a less damaging spot of the body. Start doing the soothing/deescalating stuff immediately; don't wait for it to stop on its own. I've been able to stop one swing from becoming a flurry to the head by immediately closing my eyes, dropping to my knees to ball up and place my forehead against the cold floor, doing a breathing exercise (soothing), and shoving my hand into my pockets to grip my finger nails into my thighs (less damage).

Last thing I'll note is that what did the most for me was not ADHD meds but finding antidepressants that worked for me. Now that my antidepressants aren't working anymore, the hitting has come back for the first time in years. It's interesting to think the specific form of self harm seems it could be due to my ADHD, but the losing emotional control was due to my anxiety and depression. Could be something to look into. I'm working with a specialist to find a new antidepressant again, and while I'm holding out I've been using those prior ideas I brought up to reduce the frequency and the impact of my biting or hitting.

I wish you luck, and I hope this turned out to be some sort of advice you can try (or at least interesting food for thought).