I am so guilty for smoking pot and not telling my girlfriend please help me out?

well the story goes i love her more than anything and i have been with hher over three years, welll yesterday i slept ove my buddy and he had some pot, well i smoked it and i cant tell her or she will leave me and i cant handle that i know she will leave me cause she said if i ever touched the stuff she couldnt be around me again. please how do i get rid of this guilt? i cant handle it anymore im sickk to y stomach and i just want everything to be okay and go back to normal, im extremly paranaoid i just dont want to lose her, how do i get rid of this guilt?