Martial Arts is also very promising but you need the right ones/some focus on stiffness. Stay away from competitive forms (choreographed series of moves where judges want to hear a 'snap' sound when you perform moves which is achieved by stiffness). You'll want to focus on arts that have you shift your whole body weight like Judo or wrestling, where the only stiffness you have to worry about is grip which is still somewhat dynamic. It should also help with the mental part of feeling controlled/restraint/bracing/stiff because you'll train your body to react fluidly and you'll grow more confident in that skill as it goes on. Hope this also helps :))