ZeOwl
If I had such a power, I would probably first make myself gain resistance to pain, in small amounts, I'm not a sadistic lunatic who would self-injure for that, as well as punish myself every day, hit myself with some force, and when I gain resistance, go on and on until I can take it all well and then move on to improving myself in other more violent aspects since I can take all the pain well.
I'd probably see how far I could go, drown myself, deprive myself of oxygen, Stop eating, maybe even indulge in drugs just to see what would happen ngl.
ZeOwl:my first step would likely be stabbing myself, likely immediately jump into the deep end.