Working miracles at this point in time wouldn’t necessarily cost the person their life. Jesus had to die for our sins, but a person now wouldn’t have to.
Of course all power comes from God, not the individual here.
Repenting from sin entails suffering at the best. We begin weakly and are limited in what we can do.
Trying to do too much too soon would end in nothing lasting being done at all. It would kill the progress, not our mortal lives.