What Causes Inability To Pull Foreskin Back And Purplish Penis Head?

hi
i was wondering about the skin on my penis .. i heard its suppposed to be pulled back before sex but ive never been able to do this. ive been doing like strtching excercises and i can get it so far but then it looks like the tip is going purplish like its being starved of blood so i stop. is this normal or am i just doing it too slowly?