Spits and cuts from shaving bum 😞😞

no one laugh, im italian chick and suffer from dark hair everywhere and have been bullied before but I'm older now. Fine with hair but I have a hairy bum and back, I'm very sexually actively now also so very self concious. And I don't like it anyway, I'm 19. I used a shear butter razor everywhere on my body and was so soft I did it on my bottom and it's caused lots of spots and cuts on my bum. How can I remove hair of my bum with out causing all these marks and cuts